Semiconductor device and method for manufacturing the same

ABSTRACT

A transistor in which the state of an interface between an oxide semiconductor layer and an insulating film in contact with the oxide semiconductor layer is favorable and a method for manufacturing the transistor are provided. Nitrogen is added to the vicinity of the interface between the oxide semiconductor layer and the insulating film (gate insulating layer) in contact with the oxide semiconductor layer so that the state of the interface of the oxide semiconductor layer becomes favorable. Specifically, the oxide semiconductor layer has a concentration gradient of nitrogen, and a region containing much nitrogen is provided at the interface with the gate insulating layer. A region having high crystallinity can be formed in the vicinity of the interface with the oxide semiconductor layer by addition of nitrogen, whereby the interface state can be stable.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device including anoxide semiconductor and a method for manufacturing the semiconductordevice.

In this specification, a semiconductor device generally means a devicewhich can function by utilizing semiconductor characteristics, and anelectrooptic device, a semiconductor circuit, and an electronic deviceare all semiconductor devices.

2. Description of the Related Art

In recent years, techniques to form thin film transistors (TFTs) using asemiconductor thin film (with a thickness of approximately severalnanometers to several hundreds of nanometers) which is formed over asubstrate having an insulating surface have attracted attention. Thinfilm transistors are widely applied to electronic devices such as ICs orelectro-optical devices, and are particularly expected to be rapidlydeveloped as switching elements of image display devices. Various metaloxides are used for a variety of applications. Indium oxide is awell-known material and is used as a light-transmitting electrodematerial which is necessary for liquid crystal displays and the like.

Some metal oxides have semiconductor characteristics. Examples of suchmetal oxides having semiconductor characteristics include tungstenoxide, tin oxide, indium oxide, and zinc oxide. Thin film transistors ineach of which a channel formation region is formed using such metaloxide having semiconductor characteristics have already been known(Patent Documents 1 and 2).

REFERENCE

-   [Patent Document 1] Japanese Published Patent Application No.    2007-123861-   [Patent Document 2] Japanese Published Patent Application No.    2007-096055

SUMMARY OF THE INVENTION

Electric characteristics of a transistor are easily affected by thestate of an interface between an oxide semiconductor layer and aninsulating film in contact with the oxide semiconductor layer. When theinterface between an oxide semiconductor layer and an insulating film,that is, the interface of the oxide semiconductor layer, which is on agate electrode side, is amorphous, electric characteristics of atransistor might be degraded while the transistor is manufactured orafter the transistor is manufactured.

Thus, an object of one embodiment of the present invention is to providea transistor in which the state of an interface between an oxidesemiconductor layer and an insulating film in contact with the oxidesemiconductor layer, that is, the interface of the oxide semiconductorlayer, which is on a gate electrode side, is favorable; and a method formanufacturing the transistor.

Further, another object of one embodiment of the present invention is toprovide a transistor with small variation in electric characteristicsand a method for manufacturing the transistor.

Nitrogen is added to an insulating film so that the state of aninterface between an oxide semiconductor layer and the insulating filmin contact with the oxide semiconductor layer becomes favorable. Then,the insulating film to which nitrogen is added and the oxidesemiconductor layer are provided in contact with each other, whereby aregion having high crystallinity can be formed in the vicinity of theinterface between the oxide semiconductor layer and the insulating filmin contact with the oxide semiconductor layer. Accordingly, theinterface state can be stable.

According to one embodiment of the present invention, a semiconductordevice includes a gate electrode layer; a first insulating layer incontact with the gate electrode layer; an oxide semiconductor layer incontact with the first insulating layer; and a second insulating layerin contact with the oxide semiconductor layer. In the semiconductordevice, the oxide semiconductor layer has a peak of a nitrogenconcentration (highest concentration point) at an interface with thefirst insulating layer, and the first insulating layer contains nitrogenand has a peak of a nitrogen concentration at an interface with theoxide semiconductor layer.

In the semiconductor device having the above structure, the oxidesemiconductor layer has a concentration gradient of nitrogen, whichbecomes higher as closer to the first insulating layer, and has aconcentration gradient of oxygen, which becomes higher as closer to thesecond insulating layer.

In the semiconductor device having the above structure, a region of theoxide semiconductor layer, which is in the vicinity of the interfacewith the first insulating layer, has higher crystallinity than otherregions. Note that the oxide semiconductor layer is non-single-crystal,and the oxide semiconductor layer is not entirely in an amorphous statebut includes at least a c-axis aligned crystal in the oxidesemiconductor layer.

In the semiconductor device having the above structure, a nitrogenconcentration of the oxide semiconductor layer in the vicinity of theinterface with the first insulating layer is higher than or equal to5×10¹⁹/cm³ and lower than 7 atomic %. Further, a nitrogen concentrationof the oxide semiconductor layer in the vicinity of the interface withthe second insulating layer is higher than or equal to 1×10¹⁷/cm³ andlower than 5×10¹⁹/cm³. In the oxide semiconductor layer, the energy gapof a region containing much nitrogen is smaller than the energy gap of aregion containing less nitrogen; thus, carriers can easily flow in theregion. Thus, a structure is employed for a transistor, in which muchnitrogen is contained in a region of an oxide semiconductor layer, wherecarriers flow, and less nitrogen is contained in other regions.

In the semiconductor device having the above structure, the oxidesemiconductor layer has a peak of an oxygen concentration at aninterface with the second insulating layer, and the second insulatinglayer contains oxygen and has a peak of an oxygen concentration at aninterface with the oxide semiconductor layer.

According to another embodiment of the present invention, in a methodfor manufacturing a semiconductor device, an oxide semiconductor layeris formed; a source electrode layer and a drain electrode layer areformed over the oxide semiconductor layer; nitrogen is added to part ofthe oxide semiconductor layer by plasma treatment with a N₂ or N₂O gasafter heating is performed under a reduced pressure; a gate insulatinglayer is formed over the oxide semiconductor layer, the source electrodelayer, and the drain electrode layer without exposure to air after theplasma treatment; and a gate electrode layer is formed in a positionoverlapping a region of the oxide semiconductor layer, to which nitrogenis added, through the gate insulating layer.

According to another embodiment of the present invention, in a methodfor manufacturing a semiconductor device, a gate electrode layer isformed; a gate insulating layer is formed over the gate electrode layer;nitrogen is added to part of the gate insulating layer by plasmatreatment with a N₂ or N₂O gas; an oxide semiconductor layer is formedin a position overlapping the gate electrode layer through the gateinsulating layer; a source electrode layer and a drain electrode layerare formed over the oxide semiconductor layer; oxygen is added to partof the oxide semiconductor layer by oxygen plasma treatment afterheating is performed under a reduced pressure; and an insulating layercovering the oxide semiconductor layer, the source electrode layer, andthe drain electrode layer is formed under an atmosphere containingoxygen by a sputtering method.

Note that in this specification, a hexagonal crystal structure, whichincludes trigonal and hexagonal crystal structures classified in sevencrystal systems, refers to six crystal families.

In the oxide semiconductor layer, the crystallinity of a regioncontaining nitrogen is high, interface states due to dangling bonds atthe interface between the oxide semiconductor layer and the insulatingfilm in contact with the oxide semiconductor layer are reduced, and afavorable condition of the interface can be realized.

When the condition of the interface of the oxide semiconductor layer andthe insulating film in contact with the oxide semiconductor layer ismade favorable, a transistor with higher electric characteristics can beobtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C are cross-sectional views of one embodiment of thepresent invention.

FIGS. 2A to 2C are cross-sectional views of one embodiment of thepresent invention.

FIGS. 3A to 3C are examples of a model diagram of a concentrationprofile of one embodiment of the present invention.

FIGS. 4A and 4B are a cross-sectional view of one embodiment of thepresent invention and an example of a model diagram of a concentrationprofile of one embodiment of the present invention, respectively.

FIGS. 5A to 5C are a block diagram and equivalent circuit diagrams ofone embodiment of the present invention.

FIGS. 6A to 6C are cross-sectional views of one embodiment of thepresent invention.

FIGS. 7A to 7D each illustrate one embodiment of an electronic device.

FIGS. 8A to 8C are cross-sectional views of one embodiment of thepresent invention.

FIG. 9 is an example of a model diagram of a concentration profile ofone embodiment of the present invention.

FIG. 10 illustrates a wurtzite crystal structure.

FIGS. 11A and 11B illustrate a wurtzite crystal structure.

FIGS. 12A to 12C illustrate a wurtzite crystal structure and anon-wurtzite crystal structure.

FIGS. 13A and 13B are real observation images of HAADF-STEM of a crystalstructure according to Embodiment.

FIGS. 14A and 14B are real observation images of HAADF-STEM of a crystalstructure according to Embodiment.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described in detail belowwith reference to the accompanying drawings. However, the presentinvention is not limited to the description below, and it is easilyunderstood by those skilled in the art that modes and details disclosedherein can be modified in various ways without departing from the spiritand the scope of the present invention. Therefore, the present inventionis not construed as being limited to description of the embodiments.

Embodiment 1

In this embodiment, an example of a method for manufacturing a top-gatetransistor in which the condition of an interface between an oxidesemiconductor layer and an insulating layer in contact with part of theoxide semiconductor layer is favorable will be described with referenceto FIGS. 1A to 1C.

As illustrated in FIG. 1A, a base insulating layer 101 is formed over asubstrate 100.

As the substrate 100, a substrate of a glass material such asaluminosilicate glass, aluminoborosilicate glass, or barium borosilicateglass is used. In the mass production, a mother glass with the followingsize is preferably used for the substrate 100: the 8-th generation (2160mm×2460 mm); the 9-th generation (2400 mm×2800 mm, or 2450 mm×3050 mm);the 10-th generation (2950 mm×3400 mm); or the like. The mother glassdrastically shrinks when the treatment temperature is high and thetreatment time is long. Thus, in the case where mass production isperformed with the use of the mother glass, the preferable heatingtemperature in the manufacturing process is lower than or equal to 600°C., more preferably lower than or equal to 450° C.

The base insulating layer 101 can be formed to have a single-layerstructure or a stacked-layer structure using one or more of siliconoxide, silicon oxynitride, silicon nitride oxide, aluminum oxide,aluminum oxynitride, gallium oxide, hafnium oxide, yttrium oxide, andthe like by a CVD method, a sputtering method, or the like.

Next, an oxide semiconductor layer is formed over the base insulatinglayer 101. The oxide semiconductor layer is formed using any one ofsputtering apparatuses such as an AC sputtering apparatus, a DCsputtering apparatus, and an RF sputtering apparatus. The oxidesemiconductor layer is formed by a sputtering method under a mixedatmosphere containing an oxygen gas and a rare gas, a mixed atmospherecontaining oxygen and nitrogen, and an atmosphere containing only a raregas. For the oxide semiconductor layer, an indium oxide, a tin oxide, azinc oxide; a two-component metal oxide such as an In—Zn-based oxide, aSn—Zn-based oxide, an Al—Zn-based oxide, a Zn—Mg-based oxide, aSn—Mg-based oxide, an In—Mg-based oxide, or an In—Ga-based oxide; athree-component metal oxide such as a Sn—Ga—Zn-based oxide, anAl—Ga—Zn-based oxide, a Sn—Al—Zn-based oxide, an In—Ga—Zn-based oxide(also referred to as IGZO), an In—Al—Zn-based oxide, an In—Sn—Zn-basedoxide, an In—Hf—Zn-based oxide, an In—La—Zn-based oxide, anIn—Ce—Zn-based oxide, an In—Pr—Zn-based oxide, an In—Nd—Zn-based oxide,an In—Sm—Zn-based oxide, an In—Eu—Zn-based oxide, an In—Gd—Zn-basedoxide, an In—Tb—Zn-based oxide, an In—Dy—Zn-based oxide, anIn—Ho—Zn-based oxide, an In—Er—Zn-based oxide, an In—Tm—Zn-based oxide,an In—Yb—Zn-based oxide, or an In—Lu—Zn-based oxide; or a four-componentmetal oxide such as an In—Sn—Ga—Zn-based oxide, an In—Hf—Ga—Zn-basedoxide, an In—Al—Ga—Zn-based oxide, an In—Sn—Al—Zn-based oxide, anIn—Sn—Hf—Zn-based oxide, or an In—Hf—Al—Zn-based oxide can be used.

Note that here, for example, an “In—Ga—Zn-based oxide” means an oxidecontaining In, Ga, and Zn as main components and there is no particularlimitation on the ratio of In, Ga, and Zn. The In—Ga—Zn-based oxide maycontain a metal element other than the In, Ga, and Zn.

For example, an In—Ga—Zn-based oxide with an atomic ratio ofIn:Ga:Zn=1:1:1 (=1/3:1/3:1/3) or In:Ga:Zn=2:2:1 (=2/5:2/5:1/5), or anoxide with an atomic ratio close to the above atomic ratios can be used.Alternatively, an In—Sn—Zn-based oxide with an atomic ratio ofIn:Sn:Zn=1:1:1 (=1/3:1/3:1/3), In:Sn:Zn=2:1:3 (=1/3:1/6:1/2), orIn:Sn:Zn=2:1:5 (=1/4:1/8:5/8), or an oxide with an atomic ratio close tothe above atomic ratios may be used.

However, without limitation to the materials given above, a materialwith an appropriate composition may be used depending on neededsemiconductor characteristics (e.g., mobility, threshold voltage, andvariation). In order to obtain the needed semiconductor characteristics,it is preferable that the carrier density, the impurity concentration,the defect density, the atomic ratio between a metal element and oxygen,the interatomic distance, the density, and the like be set toappropriate values.

The oxide semiconductor may be either single crystal ornon-single-crystal. In the latter case, the oxide semiconductor may beeither amorphous or polycrystal. Further, the oxide semiconductor mayhave either an amorphous structure including a portion havingcrystallinity or a non-amorphous structure.

In addition, at the formation of the oxide semiconductor layer, thepressure of a treatment chamber in a sputtering apparatus is set to lessthan 0.4 Pa, whereby mixing of an impurity such as alkali metal orhydrogen to a surface where a film is formed or a film to be formed canbe suppressed. Note that hydrogen may be contained in the film to beformed as a hydrogen molecule, water, a hydroxyl group, or hydride insome cases in addition to a hydrogen atom.

In addition, at the formation of the oxide semiconductor layer, thedistance between a target and the substrate (a T-S distance) is set togreater than or equal to 40 mm and less than or equal to 300 mm(preferably, greater than or equal to 60 mm).

In addition, at the formation of the oxide semiconductor layer by asputtering method, the temperature of the surface where a film is formedis higher than or equal to 150° C. and lower than or equal to 450° C.,preferably higher than or equal to 250° C. and lower than or equal to320° C. The temperature at which entry of impurities such as water orhydrogen into a film to be formed is prevented and the impurity isreleased to a vapor phase in the chamber is 250° C. In addition, theupper limit of the temperature of the surface where a film is formed inthe film formation by a sputtering method is the upper limit of the heattreatment temperature for the substrate or the upper limit of thetemperature of the film to be formed (if the temperature exceeds thelatter upper limit, components in the film significantly change).

Further, when the leakage rate of the treatment chamber of thesputtering apparatus is set to less than or equal to 1×10⁻¹⁰Pa·m³/second at the formation of the oxide semiconductor layer, entry ofan impurity such as an alkali metal or hydride into the oxidesemiconductor layer that is being formed by a sputtering method can bereduced. Furthermore, with the use of an entrapment vacuum pump (e.g., acryopump) as an evacuation system, counter flow of an impurity such asan alkali metal, a hydrogen atom, a hydrogen molecule, water, a hydroxylgroup, or hydride from the evacuation system can be reduced.

Alternatively, at the formation of the oxide semiconductor layer, thefilm formation may be performed by introducing, for example, a nitrogengas, an oxygen gas, an argon gas, or the like to the treatment chamberof the sputtering apparatus with the gas being heated.

Further, preheat treatment may be performed before formation of theoxide semiconductor layer, in order to remove moisture or hydrogen whichremains on an inner wall of the sputtering apparatus, on a surface ofthe target, or inside the target material. As the preheat treatment, amethod in which the inside of a film formation chamber is heated to 200°C. to 600° C. under a reduced pressure, a method in which introductionand evacuation of nitrogen or an inert gas are repeated while the insideof a film formation chamber is heated, and the like can be given. Inthis case, not water but oil or the like is preferably used as a coolantfor the target. Although a certain level of advantageous effect can beobtained when introduction and evacuation of nitrogen are repeatedwithout heating, it is more preferable to perform the treatment with theinside of the film formation chamber heated.

The base insulating layer 101 and the oxide semiconductor layer arepreferably formed in succession without exposure to the air. With thesuccessive formation, the respective interfaces of the stacked layerscan be formed without being contaminated by an atmospheric component ora contamination impurity element floating in the air.

After the oxide semiconductor layer is formed, if necessary, heattreatment may be performed under an atmosphere which hardly containshydrogen and moisture (a nitrogen atmosphere, an oxygen atmosphere, adry-air atmosphere (e.g., as for moisture, a dew point is lower than orequal to −40° C., preferably lower than or equal to −60° C.), or thelike) at a temperature higher than or equal to 150° C. and lower than orequal to 650° C., preferably a temperature higher than or equal to 200°C. and lower than or equal to 500° C. This heat treatment can be calleddehydration or dehydrogenation, which is for detaching H, OH, or thelike from the oxide semiconductor layer.

Next, the oxide semiconductor layer is processed, so that an island-likeoxide semiconductor layer 102 is formed. The oxide semiconductor layercan be processed as follows: a mask having a desired shape is formedover the oxide semiconductor layer by a photolithography technique or anink jet method; and the oxide semiconductor layer is selectively etchedusing the mask.

For the etching of the oxide semiconductor layer, either wet etching ordry etching may be employed. It is needless to say that both of them maybe employed in combination.

Next, a conductive film is formed over the oxide semiconductor layer102. The conductive film can be formed by a sputtering method or avacuum evaporation method. As a material used for the conductive film, ametal material such as Al, Cr, Cu, Ta, Ti, Mo, and W, or an alloymaterial containing the metal material as a component is used. Further,a structure may be employed in which a layer of a high-melting pointmetal such as Cr, Ta, Ti, Mo, or W is stacked on a lower side and/or anupper side of a metal layer of Al, Cu, or the like. Furthermore, an Almaterial to which an element which prevents generation of hillocks orwhisker in an Al film, such as Si, Ti, Ta, W, Mo, Cr, Nd, Sc, or Y isadded may be used, leading to improvement in heat resistance.

For example, the metal conductive film preferably has a three-layerstructure in which an aluminum layer is stacked over a titanium layerand a titanium layer is stacked over the aluminum layer, or a threelayer structure in which an aluminum layer is stacked over a molybdenumlayer and a molybdenum layer is stacked over the aluminum layer.Alternatively, the metal conductive film can have a two-layer structurein which an aluminum layer and a tungsten layer are stacked, or atwo-layer structure in which an aluminum layer and a molybdenum layerare stacked. Needless to say, the metal conductive film may have asingle-layer structure or a stacked-layer structure including four ormore layers. Note that in the case where copper is used as one of thematerials of the metal conductive film, the following stack may be used:an alloy layer of copper, magnesium, and aluminum is provided in contactwith the oxide semiconductor layer; and a copper layer is provided incontact with the alloy layer of copper, magnesium, and aluminum.

Next, a source electrode layer 103 or a drain electrode layer 104 can beformed in such a manner that a mask having a desired shape is formedover the conductive film by a photolithography technique or an ink jetmethod, and the conductive film is selectively etched using the mask.Note that at the etching, part of the oxide semiconductor layer 102 maybe etched.

As illustrated in FIG. 1B, after the source electrode layer 103 or thedrain electrode layer 104 is formed, heat treatment is performed under areduced pressure. By the heat treatment under a reduced pressure,excessive hydrogen (including water and a hydroxyl group) contained inthe oxide semiconductor layer 102 can be removed. After the heattreatment under a reduced pressure is performed, plasma treatment usinga gas such as N₂O or N₂ is performed. Nitrogen is added to the vicinityof the surface of the exposed oxide semiconductor layer 102 byperforming the plasma treatment. The nitrogen concentration of an oxidesemiconductor layer 102 a to which nitrogen is added by the plasmatreatment is higher than the nitrogen concentration of an oxidesemiconductor layer 102 b to which nitrogen is not added.

In the oxide semiconductor layer, the energy gap of a region containingmuch nitrogen (the oxide semiconductor layer 102 a to which nitrogen isadded) is smaller than the energy gap of a region containing lessnitrogen (the oxide semiconductor layer 102 b to which nitrogen is notadded); thus, carriers easily flow through the region. Much nitrogen iscontained in a region of the oxide semiconductor layer, where carriersflow, and less nitrogen is contained in other regions by adding nitrogento the vicinity of the surface of the exposed oxide semiconductor layerby the plasma treatment.

Note that the plasma treatment using a gas such as N₂O or N₂ may beperformed before the source electrode layer or the drain electrode layeris formed in the film formation chamber of the sputtering apparatus,where the oxide semiconductor layer has been formed; or may be performedbefore a gate insulating layer is formed in a plasma CVD apparatus afterthe source electrode layer or the drain electrode layer is formed.

The oxide semiconductor layer 102 a to which nitrogen is added hashigher crystallinity than the oxide semiconductor layer 102 b to whichnitrogen is not added, and is c-axis aligned. Further, in the oxidesemiconductor layer 102 a to which nitrogen is added, the uniformity ofthe exposed surface region is increased.

A crystal contained in the oxide semiconductor layer 102 a to whichnitrogen is added has a hexagonal wurtzite crystal structure, and acrystal contained in the oxide semiconductor layer 102 b to whichnitrogen is not added has a hexagonal non-wurtzite crystal structure.Since both the wurtzite crystal structure and the non-wurtzite crystalstructure are hexagonal crystal structures, a hexagonal lattice imagecan be seen from the c-axis direction.

As illustrated in FIG. 1C, after the plasma treatment using a gas suchas N₂O or N₂ is performed, a gate insulating layer 105 which covers thesource electrode layer 103 or the drain electrode layer 104 and is incontact with part of the oxide semiconductor layer 102 is formed withoutexposure to the air. The gate insulating layer 105 can be formed to havea single-layer structure or a stacked-layer structure using one or moreof silicon oxide, silicon oxynitride, silicon nitride oxide, aluminumoxide, aluminum oxynitride, gallium oxide, hafnium oxide, yttrium oxide,and the like by a CVD method, a sputtering method, or the like. Notethat a silicon oxynitride film (also referred to as SiO_(x)N_(y)(x>y>0)) is more preferable as the gate insulating layer 105.

Note that in the case where residual nitrogen enters part of the gateinsulating layer 105 or the entire gate insulating layer 105, the filmquality of the gate insulating layer 105 might be degraded. Thus, theplasma treatment using a gas such as N₂O or N₂ and the film formation ofthe gate insulating layer 105 may be performed in separate filmformation chambers using a multi-chamber film formation apparatus or thelike.

Next, a gate electrode layer 106 is formed over the gate insulatinglayer 105 so as to overlap the oxide semiconductor layer 102. The gateelectrode layer 106 can be formed using a metal material such astitanium, molybdenum, chromium, tantalum, tungsten, copper, or aluminum,an alloy material thereof, or the like.

Between the gate electrode layer 106 and the gate insulating layer 105,an In—Ga—Zn—O film containing nitrogen, an In—Sn—O film containingnitrogen, an In—Ga—O film containing nitrogen, an In—Zn—O filmcontaining nitrogen, a Sn—O film containing nitrogen, an In—O filmcontaining nitrogen, or a metal nitride film (InN, ZnN, or the like) ispreferably provided as a material layer in contact with the gateinsulating layer. These films each have a work function of 5 eV orhigher, preferably 5.5 eV or higher; thus, the threshold voltage of theelectric characteristics of the transistor can be positive. Accordingly,a so-called normally-off switching element can be obtained. For example,in the case where an In—Ga—Zn—O film containing nitrogen is used, anIn—Ga—Zn—O film in which the nitrogen concentration is higher than orequal to 1×10²⁰/cm³ and lower than 7 atomic % and at least higher thanthat the nitrogen concentration of the oxide semiconductor layer 102 ais used.

The gate electrode layer 106 can be formed in such a manner that aconductive film is formed over the gate insulating layer 105, a maskhaving a desired shape is formed over the conductive film by aphotolithography technique or an ink jet method, and the conductive filmformed over the gate insulating layer 105 is selectively etched usingthe mask.

Through the above process, a top-gate transistor 111 in which thecondition of the interface between the oxide semiconductor layer and theinsulating layer in contact with part of the oxide semiconductor layeris favorable can be manufactured.

Next, the state of the nitrogen concentration of the oxide semiconductorlayer in the vicinity of the interface with the insulating layer incontact with the oxide semiconductor layer will be described in detailwith reference to the nitrogen concentration profile illustrated in FIG.3A. FIG. 3A is a schematic diagram illustrating the nitrogenconcentration profile in the film thickness direction of a cross sectiontaken along a dotted line in FIG. 1C. A first insulating layer and asecond insulating layer in FIG. 3A correspond to the gate insulatinglayer and the base insulating layer in FIG. 1C, respectively.

As illustrated in FIG. 3A, the value of the nitrogen concentration ofthe oxide semiconductor layer 102 b to which nitrogen is not added doesnot fluctuate much and is maintained constant. The nitrogenconcentration of the oxide semiconductor layer 102 a to which nitrogenis added is higher as closer to the interface between the oxidesemiconductor layer and the first insulating layer. The nitrogenconcentration has a peak at the interface between the oxidesemiconductor layer and the first insulating layer.

Further, the nitrogen concentration of the first insulating layer is thehighest at the interface with the oxide semiconductor layer, isgradually reduced at positions farther from the oxide semiconductorlayer, and is reduced more greatly in a certain region.

The nitrogen concentration fluctuates in the vicinity of the interfacebetween the oxide semiconductor layer 102 a to which nitrogen is addedand the oxide semiconductor layer 102 b to which nitrogen is not added.The nitrogen concentration has such a concentration gradient due to thedegree of crystallinity. The oxide semiconductor layer 102 a to whichnitrogen is added by the plasma treatment has higher crystallinity thanthe oxide semiconductor layer 102 b to which nitrogen is not added. Thatis, the interface between the oxide semiconductor layer 102 a to whichnitrogen is added and the oxide semiconductor layer 102 b to whichnitrogen is not intentionally added is an interface between a regionhaving high crystallinity and a region having low crystallinity. As aresult, the nitrogen concentration fluctuates easily because theinterface is relatively distinct.

Specifically, the nitrogen concentration of a region 112 in the vicinityof the interface between the first insulating layer and the oxidesemiconductor layer is higher than or equal to 5×10¹⁹/cm³ and lower than7 atomic %.

Further, specifically, the nitrogen concentration of the oxidesemiconductor layer 102 b to which nitrogen is not added by the plasmatreatment is lower than 5×10¹⁹/cm³, preferably higher than or equal to1×10¹⁷/cm³ and lower than 5×10¹⁹/cm³.

The nitrogen concentration has a peak at the interface between the oxidesemiconductor layer and the first insulating layer; therefore, thecrystallinity is the highest at the interface. As a result, interfacestates due to dangling bonds at the interface between the oxidesemiconductor layer and the first insulating layer are reduced;therefore, a favorable condition of the interface can be realized. Thus,as compared to the case where the interface between the oxidesemiconductor layer and the first insulating layer is in an amorphousstate, the electric characteristics of the transistor can be preventedfrom being degraded.

In the transistor 111 having the nitrogen concentration profileillustrated in FIG. 3A, the nitrogen concentration of the region 112 inthe vicinity of the interface between the first insulating layer and theoxide semiconductor layer is high. Thus, the region 112 in the vicinityof the interface has higher crystallinity than the crystallinity ofother regions and is also c-axis aligned. Therefore, the transistor 111with higher electric characteristics (e.g., the field-effect mobilityand the threshold value) can be obtained. The region 112 in the vicinityof the interface also has higher uniformity than other regions;therefore, the transistor 111 with small variation in electriccharacteristics can be obtained.

Further, the transistor 111 is manufactured in which a channel region isincluded in a crystalline oxide semiconductor film having an a-b planewhere bonds for forming hexagonal lattices are formed, which issubstantially parallel to the plan surface of the substrate, and c-axeswhich are substantially perpendicular to the plan surface of thesubstrate, whereby the amount of fluctuation in the threshold voltage ofthe transistor 111 between before and after performance of abias-thermal stress (BT) test or light irradiation test on thetransistor 111 can be reduced. Accordingly, the transistor 111 withstable electric characteristics can be manufactured.

Embodiment 2

In this embodiment, an example of a method for manufacturing abottom-gate transistor in which the condition of an interface between anoxide semiconductor layer and an insulating layer in contact with partof the oxide semiconductor layer is favorable will be described withreference to FIGS. 2A to 2C.

As illustrated in FIG. 2A, a gate electrode layer 201 is formed over asubstrate 200.

The gate electrode layer 201 can be formed using a metal material suchas titanium, molybdenum, chromium, tantalum, tungsten, aluminum, orcopper, an alloy material thereof, or the like. The gate electrode layer201 can be formed in such a manner that a conductive film is formed overthe substrate 200 by a sputtering method or a vacuum evaporation method,a mask is formed over the conductive film by a photolithographytechnique or an ink jet method, and the conductive film is etched usingthe mask.

Next, a gate insulating layer 202 covering the gate electrode layer 201is formed. Note that a silicon oxynitride film (also referred to asSiO_(x)N_(y) (x>y>0)) is more preferable as the gate insulating layer202. After the gate insulating layer 202 is formed, plasma treatmentusing a gas such as N₂O or N₂ is performed. Nitrogen is added to thevicinity of the surface of the gate insulating layer 202 by performingthe plasma treatment. The nitrogen concentration of a gate insulatinglayer 202 a to which nitrogen is added is higher than the nitrogenconcentration of a gate insulating layer 202 b to which nitrogen is notadded.

Between the gate electrode layer 201 and the gate insulating layer, anIn—Ga—Zn—O film containing nitrogen, an In—Sn—O film containingnitrogen, an In—Ga—O film containing nitrogen, an In—Zn—O filmcontaining nitrogen, a Sn—O film containing nitrogen, an In—O filmcontaining nitrogen, a metal nitride film (InN, ZnN, or the like) ispreferably provided as a material layer in contact with the gateinsulating layer. These films each have a work function of 5 eV orhigher, preferably 5.5 eV or higher; thus, the threshold voltage of theelectric characteristics of the transistor can be positive. Accordingly,a so-called normally-off switching element can be obtained. For example,in the case where an In—Ga—Zn—O film containing nitrogen is used, anIn—Ga—Zn—O film in which the nitrogen concentration higher than at leastthat of the oxide semiconductor layer 102, specifically an In—Ga—Zn—Ofilm in which the nitrogen concentration is higher than or equal to 7atomic % is used.

Note that in the case where residual nitrogen enters part of the gateinsulating layer 202 or the entire gate insulating layer 202, the filmquality of the gate insulating layer 202 might be degraded. Thus, theplasma treatment using a gas such as N₂O or N₂ and the film formation ofthe gate insulating layer 202 may be performed in separate filmformation chambers using a multi-chamber film formation apparatus or thelike.

Note that the plasma treatment using a gas such as N₂O or N₂ may beperformed in a plasma CVD apparatus where the gate insulating layer hasbeen formed; or may be performed before an oxide semiconductor layer isformed in a film formation chamber of a sputtering apparatus, where theoxide semiconductor layer is to be formed later.

Next, as illustrated in FIG. 2B, an oxide semiconductor layer is formedover and in contact with the gate insulating layer 202. The oxidesemiconductor layer is formed by a sputtering method under a mixedatmosphere containing an oxygen gas and a rare gas, a mixed atmospherecontaining oxygen and nitrogen, and an atmosphere containing only a raregas.

Next, the oxide semiconductor layer is processed, so that an island-likeoxide semiconductor layer 203 is formed. The oxide semiconductor layercan be processed as follows: a mask having a desired shape is formedover the oxide semiconductor layer by a photolithography technique or anink jet method; and the oxide semiconductor layer is selectively etchedusing the mask.

In such a manner, the oxide semiconductor layer 203 is provided incontact with the gate insulating layer 202 a to which nitrogen is added,whereby an oxide semiconductor layer 207 having high crystallinity canbe formed also in the vicinity of the interface of the oxidesemiconductor layer 203 in contact with the gate insulating layer 202 ato which nitrogen is added.

In the oxide semiconductor layer, the energy gap of a region containingmuch nitrogen (the oxide semiconductor layer 207 having highcrystallinity) is smaller than the energy gap of a region containingless nitrogen; thus, carriers easily flow through the region. Muchnitrogen is contained in a region of the oxide semiconductor layer,where carriers flow, and less nitrogen is contained in other regions byadding nitrogen to the vicinity of the surface of the gate insulatinglayer by the plasma treatment.

A crystal contained in the oxide semiconductor layer 207 having highcrystallinity has a hexagonal wurtzite crystal structure.

That is, a region 212 in the vicinity of the interface between the gateinsulating layer 202 a to which nitrogen is added and the oxidesemiconductor layer 203 has higher crystallinity than other regions.Further, the region 212 in the vicinity of the interface has higheruniformity than other regions. Furthermore, interface states due todangling bonds at the interface between the gate insulating layer 202and the oxide semiconductor layer 203 are reduced; therefore, afavorable condition of the interface can be realized.

Next, a conductive film is formed over the oxide semiconductor layer203. The conductive film can be formed by a sputtering method or avacuum evaporation method. As a material used for the conductive film, ametal material such as Al, Cu, Cr, Ta, Ti, Mo, and W, or an alloymaterial containing the metal material as a component is used. Further,a structure may be employed in which a layer of a high-melting pointmetal such as Cr, Ta, Ti, Mo, or W is stacked on a lower side and/or anupper side of a metal layer of Al, Cu, or the like.

Next, a source electrode layer 204 or a drain electrode layer 205 can beformed in such a manner that a mask having a desired shape is formedover the conductive film by a photolithography technique or an ink jetmethod, and the conductive film is selectively etched using the mask.Note that at the etching, part of the oxide semiconductor layer 203 maybe etched.

After the source electrode layer 204 or the drain electrode layer 205 isformed, heat treatment is performed under a reduced pressure. By theheat treatment under a reduced pressure, excessive hydrogen (includingwater and a hydroxyl group) contained in the oxide semiconductor layer203 can be removed. After the heat treatment under a reduced pressure isperformed, oxygen plasma treatment is performed. Oxygen is added to thevicinity of the surface of the exposed oxide semiconductor layer 203 byperforming the oxygen plasma treatment. The oxygen concentration of anoxide semiconductor layer 203 a to which oxygen is added is higher thanthe oxygen concentration of an oxide semiconductor layer 203 b to whichoxygen is not added.

As illustrated in FIG. 2C, after the oxygen plasma treatment isperformed, a protective insulating layer 206 which covers the sourceelectrode layer 204 or the drain electrode layer 205 and is in contactwith part of the oxide semiconductor layer 203 is formed by a sputteringmethod. The protective insulating layer 206 from which part of containedoxygen is released by heat treatment is easily formed by a sputteringmethod, which is preferable. When the protective insulating layer 206from which part of contained oxygen is released by heat treatment isformed by a sputtering method, the amount of oxygen contained in a filmformation gas is preferably large, and oxygen, a mixed gas of oxygen anda rare gas, or the like can be used. Typically, the oxygen concentrationof a film formation gas is preferably higher than or equal to 6% andlower than or equal to 100%.

As the protective insulating layer 206, a single layer or a stackedlayer selected from silicon oxide, silicon oxynitride, silicon nitride,silicon nitride oxide, aluminum oxide, aluminum oxynitride, aluminumgallium oxide, and gallium oxide can be used.

Note that the thickness of the protective insulating layer 206 isgreater than or equal to 50 nm, preferably greater than or equal to 200nm and less than or equal to 500 nm. With the thick protectiveinsulating layer 206, the amount of oxygen released from the protectiveinsulating layer 206 can be increased, and thus the increase makes itpossible to reduce defects at the interface between the protectiveinsulating layer 206 and the oxide semiconductor layer 203.

After the protective insulating layer 206 is formed, if necessary, heattreatment may be performed under an atmosphere which hardly containshydrogen and moisture (a nitrogen atmosphere, an oxygen atmosphere, adry-air atmosphere (e.g., as for moisture, a dew point is lower than orequal to −40° C., preferably lower than or equal to −60° C.), or thelike) at a temperature higher than or equal to 150° C. and lower than orequal to 650° C., preferably a temperature higher than or equal to 200°C. and lower than or equal to 500° C.

Through the above process, a bottom-gate transistor 211 in which thecondition of the interface between the oxide semiconductor layer and theinsulating layer in contact with the oxide semiconductor layer isfavorable can be manufactured.

Next, the state of the nitrogen concentration of the oxide semiconductorlayer in the vicinity of the interface with the insulating layer incontact with the oxide semiconductor layer will be described in detailwith reference to the nitrogen concentration profile illustrated in FIG.3B. In addition, the state of the oxygen concentration of the oxidesemiconductor layer in the vicinity of the interface with the insulatinglayer in contact with the oxide semiconductor layer will be described indetail with reference to the oxygen concentration profile illustrated inFIG. 3C. FIG. 3B is a schematic diagram illustrating the nitrogenconcentration profile in the film thickness direction of a cross sectiontaken along a dotted line in FIG. 2C, and FIG. 3C is a schematic diagramillustrating the oxygen concentration profile in the film thicknessdirection of a cross section taken along a dotted line in FIG. 2C. Afirst insulating layer and a second insulating layer in each of FIGS. 3Band 3C correspond to the gate insulating layer and the protectiveinsulating layer in FIG. 2C, respectively. Note that in FIGS. 3B and 3C,the nitrogen concentration and the oxygen concentration are each adifferent concentration. The nitrogen concentration profile in FIG. 3Band the oxygen concentration profile in FIG. 3C are schematic diagramsillustrating the respective relative relations, and FIGS. 3B and 3C doesnot illustrate which of the concentrations is, for example, high or low.For the purpose of comparison of profiles, FIG. 3C includes the nitrogenconcentration profile of FIG. 3B.

As illustrated in FIG. 3B, the nitrogen concentration of the gateinsulating layer 202 a to which nitrogen is added becomes higher andbecomes gradually higher as closer to the interface between the firstinsulating layer and the oxide semiconductor layer. The nitrogenconcentration has a peak at the interface between the first insulatinglayer and the oxide semiconductor layer.

Further, the nitrogen concentration of the oxide semiconductor layer 207having high crystallinity is the highest at the interface with the firstinsulating layer, is gradually reduced at positions farther from thefirst insulating layer, and is reduced more greatly in a certain region.The value of the nitrogen concentration of the oxide semiconductor layerexcept for the oxide semiconductor layer 207 having high crystallinitydoes not fluctuate much and is maintained constant.

The oxide semiconductor layer 203 is provided in contact with the gateinsulating layer 202 a to which nitrogen is added, whereby the region212 in the vicinity of the interface between the gate insulating layer202 a to which nitrogen is added and the oxide semiconductor layer 203has higher crystallinity than other regions. As a result, the nitrogenconcentration fluctuates easily because an interface between a regionhaving high crystallinity and a region having low crystallinity isrelatively distinct.

Specifically, the nitrogen concentration of the region 212 in thevicinity of the interface between the first insulating layer and theoxide semiconductor layer is higher than or equal to 5×10¹⁹/cm³ andlower than 7 atomic %.

Further, specifically, the nitrogen concentration of the oxidesemiconductor layer other than the oxide semiconductor layer 207 havinghigh crystallinity is higher than or equal to 1×10¹⁷/cm³ and lower than5×10¹⁹/cm³.

As illustrated in FIG. 3C, the oxygen concentration of the oxidesemiconductor layer becomes gradually higher as closer to the secondinsulating layer. The oxygen concentration has a peak at the interfacebetween the second insulating layer and the oxide semiconductor layer.

Further, the oxygen concentration of the second insulating layer is thehighest in a region on the oxide semiconductor layer side, and isgradually reduced at positions farther from the oxide semiconductorlayer.

The oxygen concentration of the oxide semiconductor layer fluctuatesgradually. The oxygen concentration has such a gradual concentrationgradient due to the interface between the oxide semiconductor layer 203a to which oxygen is added and the oxide semiconductor layer 203 b towhich oxygen is not added, which is ambiguous.

The nitrogen concentration has a peak at the interface between the oxidesemiconductor layer and the first insulating layer; therefore, thecrystallinity is the highest at the interface. As a result, interfacestates due to dangling bonds at the interface between the oxidesemiconductor layer and the first insulating layer are reduced;therefore, a favorable condition of the interface can be realized. Thus,as compared to the case where the interface between the oxidesemiconductor layer and the first insulating layer is in an amorphousstate, the electric characteristics of the transistor can be preventedfrom being degraded.

In the transistor 211 having the nitrogen concentration profileillustrated in FIG. 3B, the nitrogen concentration of the region 212 inthe vicinity of the interface between the first insulating layer and theoxide semiconductor layer is high. Thus, the region 212 in the vicinityof the interface has higher crystallinity than other regions and is alsoc-axis aligned. Therefore, the transistor 211 with higher electriccharacteristics (e.g., the field-effect mobility and the thresholdvalue) can be obtained. The region 212 in the vicinity of the interfacealso has higher uniformity than other regions; therefore, the transistor211 with small variation in electric characteristics can be obtained.

Further, the transistor 211 is manufactured in which a channel region isincluded in a crystalline oxide semiconductor film having an a-b planewhere bonds for forming hexagonal lattices are formed, which issubstantially parallel to the plan surface of the substrate, and c-axeswhich are substantially perpendicular to the plan surface of thesubstrate, whereby the amount of fluctuation in the threshold voltage ofthe transistor 211 between before and after performance of abias-thermal stress (BT) test or light irradiation test on thetransistor 211 can be reduced. Accordingly, the transistor 211 withstable electric characteristics can be manufactured.

Note that as illustrated in FIGS. 4A and 4B, when an oxide semiconductorlayer 203 is formed, the oxide semiconductor layer 203 may be formed ina stack by changing film formation conditions plural times. After anoxide semiconductor layer 203 d having a high nitrogen concentration isformed, an oxide semiconductor layer 203 c may be formed over and incontact with the oxide semiconductor layer 203 d having a high nitrogenconcentration.

In this case, for example, the oxide semiconductor layer 203 d having ahigh nitrogen concentration may be formed to a thickness of greater thanor equal to 1 nm and less than or equal to 10 nm by a sputtering methodusing only a nitrogen gas, and the oxide semiconductor layer 203 c maybe formed by a sputtering method under a mixed atmosphere containing anoxygen gas and a rare gas, a mixed atmosphere containing oxygen andnitrogen, and an atmosphere containing only a rare gas. The oxidesemiconductor layer 203 d having a high nitrogen concentration and theoxide semiconductor layer 203 c can be formed by switching the kinds ofgases introduced into the film formation chamber; therefore, thenitrogen concentration and the oxygen concentration can be controlledeasily and high mass productivity can be realized.

In the oxide semiconductor layer, the energy gap of a region containingmuch nitrogen (the oxide semiconductor layer 203 d having a highnitrogen concentration) is smaller than the energy gap of a regioncontaining less nitrogen (the oxide semiconductor layer 203 c); thus,carriers easily flow through the region. Much nitrogen can be containedin a region of the oxide semiconductor layer, where carriers flow, andless nitrogen can be contained in other regions by switching the kindsof gases introduced into the film formation chamber and forming theoxide semiconductor layer in a stacked-layer structure.

A crystal contained in the oxide semiconductor layer 203 d having a highnitrogen concentration has a hexagonal wurtzite crystal structure, and acrystal contained in the oxide semiconductor layer 203 c has a hexagonalnon-wurtzite crystal structure. Since both the wurtzite crystalstructure and the non-wurtzite crystal structure are hexagonal crystalstructures, a hexagonal lattice image can be seen from the c-axisdirection.

As illustrated in FIG. 4A, in the case where the oxide semiconductorlayer 203 d having a high nitrogen concentration is provided over and incontact with the gate insulating layer 202 a to which nitrogen is added,the crystallinity of a region 214 in the vicinity of the interface canbe controlled more easily than the case where the oxide semiconductorlayer 203 is formed over and in contact with the gate insulating layer202 a to which nitrogen is added, as illustrated in FIGS. 2A to 2C.

At the formation of the oxide semiconductor layer 203 d having a highnitrogen concentration, the gas flow rate of nitrogen which isintroduced at the film formation, or the like is controlled, whereby thecontrol of the degree of crystallinity, the crystal structure, and thevarious parameters involved in the crystal structure can be changed.

Next, the state of the nitrogen concentration of the oxide semiconductorlayer in the vicinity of the interface with the insulating layer incontact with the oxide semiconductor layer can be described withreference to the nitrogen concentration profile illustrated in FIG. 4B.FIG. 4B is a schematic diagram illustrating the nitrogen concentrationprofile in the film thickness direction in a cross section taken along adotted line in FIG. 4A. A first insulating layer and a second insulatinglayer in FIG. 4B correspond to the gate insulating layer and theprotective insulating layer in FIG. 4A, respectively.

As illustrated in FIG. 4B, the value of the nitrogen concentration ofthe oxide semiconductor layer 203 c does not fluctuate much and ismaintained constant.

The nitrogen concentration of the region 214 in the vicinity of theinterface is increased stepwise. Specifically, the nitrogenconcentration of the region 214 in the vicinity of the interface isincreased in two phases as closer to the first insulating layer. It isthe interface between the oxide semiconductor layer 203 c and the oxidesemiconductor layer 203 d having a high nitrogen concentration at whichthe nitrogen concentration is increased at first. The value of thenitrogen concentration which is once increased does not fluctuate muchand is maintained constant. Further, the oxide semiconductor layer 203 dhaving a high nitrogen concentration is in contact with the gateinsulating layer 202 a to which nitrogen is added; therefore, regions inwhich the progress speed of crystallization differ is formed in theoxide semiconductor layer 203 d having a high nitrogen concentration.Thus, it is the interface between the regions in which the progressspeed of crystallization differ in the oxide semiconductor layer 203 dhaving a high nitrogen concentration at which the nitrogen concentrationis increased the second.

The progress speed of crystallization differs in the oxide semiconductorlayer 203 d having a high nitrogen concentration because the oxidesemiconductor layer 203 d having a high nitrogen concentration is not incontact with the gate insulating layer 202 b to which nitrogen is notadded but is in contact with the gate insulating layer 202 a to whichnitrogen is added. When the oxide semiconductor layer 203 d having ahigh nitrogen concentration is formed over and in contact with the gateinsulating layer 202 a to which nitrogen is added, there is a differencein the progress speed of crystallization in the oxide semiconductorlayer 203 d having a high nitrogen concentration.

As compared to the interface between the oxide semiconductor layer andthe first insulating layer in FIG. 3B, the interface between the oxidesemiconductor layer and the first insulating layer in FIG. 4B containsmuch nitrogen and thus crystallization progresses easily.

The nitrogen concentration has a peak at the interface between the oxidesemiconductor layer and the first insulating layer; therefore, thecrystallinity is the highest at the interface. In addition, the peakvalue of the nitrogen concentration is higher than the peak value of thenitrogen concentration in FIG. 3B. As a result, interface states due todangling bonds at the interface between the oxide semiconductor layerand the first insulating layer are reduced; therefore, a favorablecondition of the interface can be realized. Thus, as compared to thecase where the interface between the oxide semiconductor layer and thefirst insulating layer is in an amorphous state, the electriccharacteristics of the transistor can be prevented from being degraded.

Further, in a transistor 213 having the nitrogen concentration profileillustrated in FIG. 4B, the nitrogen concentration of the region 214 inthe vicinity of the interface between the first insulating layer and theoxide semiconductor layer is high, and further the nitrogenconcentration at the interface between the first insulating layer andthe oxide semiconductor layer is much higher. Thus, the region 214 inthe vicinity of the interface has higher crystallinity than otherregions. The region 214 in the vicinity of the interface has an a-bplane where bonds for forming hexagonal lattices are formed, which issubstantially parallel to the plan surface of the substrate, and c-axeswhich are substantially perpendicular to the plan surface of thesubstrate. Therefore, the transistor 213 with higher electriccharacteristics (e.g., the field-effect mobility and the thresholdvalue) can be obtained. The region 214 in the vicinity of the interfacealso has higher uniformity than other regions; therefore, the transistor213 with small variation in electric characteristics can be obtained.

Further, the transistor 213 is manufactured in which a channel region isincluded in a crystalline oxide semiconductor film having an a-b planewhere bonds for forming hexagonal lattices are formed, which issubstantially parallel to the plan surface of the substrate, and c-axeswhich are substantially perpendicular to the plan surface of thesubstrate, whereby the amount of fluctuation in the threshold voltage ofthe transistor 213 between before and after performance of abias-thermal stress (BT) test or light irradiation test on thetransistor 213 can be reduced. Accordingly, the transistor 213 withstable electric characteristics can be manufactured.

Embodiment 3

In this embodiment, an example of a method for manufacturing abottom-gate transistor in which the condition of an interface between anoxide semiconductor layer and an insulating layer in contact with partof the oxide semiconductor layer is favorable will be described withreference to FIGS. 8A to 8C.

As illustrated in FIG. 8A, a gate electrode layer 301 is formed over asubstrate 300.

The gate electrode layer 301 can be formed using a metal material suchas titanium, molybdenum, chromium, tantalum, tungsten, aluminum, orcopper, an alloy material thereof, or the like. The gate electrode layer301 can be formed in such a manner that a conductive film is formed overthe substrate 300 by a sputtering method or a vacuum evaporation method,a mask is formed over the conductive film by a photolithographytechnique or an ink jet method, and the conductive film is etched usingthe mask.

Next, a gate insulating layer 302 covering the gate electrode layer 301is formed. Note that a silicon oxynitride film (also referred to asSiO_(x)N_(y) (x>y>0)) is more preferable as the gate insulating layer302. After the gate insulating layer 302 is formed, plasma treatmentusing a gas such as N₂O or N₂ is performed. Nitrogen is added to thevicinity of the surface of the gate insulating layer 302 by performingthe plasma treatment. The nitrogen concentration of a gate insulatinglayer 302 a to which nitrogen is added is higher than the nitrogenconcentration of a gate insulating layer 302 b to which nitrogen is notadded.

Note that in the case where residual nitrogen enters part of the gateinsulating layer 302 or the entire gate insulating layer 302, the filmquality of the gate insulating layer 302 might be degraded. Thus, theplasma treatment using a gas such as N₂O or N₂ and the film formation ofthe gate insulating layer 302 may be performed in separate filmformation chambers using a multi-chamber film formation apparatus or thelike.

Note that the plasma treatment using a gas such as N₂O or N₂ may beperformed in a plasma CVD apparatus where the gate insulating layer hasbeen formed; or may be performed before an oxide semiconductor layer isformed in a film formation chamber of a sputtering apparatus, where theoxide semiconductor layer is to be formed later.

Next, as illustrated in FIG. 8B, an oxide semiconductor layer is formedover and in contact with the gate insulating layer 302. Note that at theformation of an oxide semiconductor layer 303, an oxide semiconductorlayer 303 d having a high nitrogen concentration and an oxidesemiconductor layer 303 c can be formed by switching the kinds of gasesintroduced into the film formation chamber. First, only a nitrogen gasis introduced into the film formation chamber and then halfway throughthe process the kinds of gases introduced into the film formationchamber is switched, whereby the oxide semiconductor layer is formed bya sputtering method under a mixed atmosphere containing an oxygen gasand a rare gas, a mixed atmosphere containing oxygen and nitrogen, andan atmosphere containing only a rare gas.

In addition, at the formation of the oxide semiconductor layer, thepressure of a treatment chamber in a sputtering apparatus is set to lessthan 0.7 Pa, preferably from 0.1 Pa to 0.5 Pa, whereby mixing of animpurity such as alkali metal or hydrogen to a surface where a film isformed or a film to be formed can be suppressed. Note that hydrogen maybe contained in the film to be formed as a hydrogen molecule, water, ahydroxyl group, or hydride in some cases, in addition to a hydrogenatom.

Further, at the formation of the oxide semiconductor layer, the filmformation is performed by introducing, for example, a nitrogen gas, anoxygen gas, an argon gas, or the like to the treatment chamber of thesputtering apparatus with the gas being heated. Crystallization canprogress gradually from a region in contact with the gate insulatinglayer 302 by performing the film formation during the heating. Thus,with the formation by sputtering performed only once, consequently, theoxide semiconductor layer 303 d having a high nitrogen concentration andthe oxide semiconductor layer 303 c can be stacked.

Next, the oxide semiconductor layer is processed, so that an island-likeoxide semiconductor layer 303 is formed. The oxide semiconductor layercan be processed as follows: a mask having a desired shape is formedover the oxide semiconductor layer by a photolithography technique or anink jet method; and the oxide semiconductor layer is selectively etchedusing the mask.

In the oxide semiconductor layer, the energy gap of a region containingmuch nitrogen (the oxide semiconductor layer 303 d having a highnitrogen concentration) is smaller than the energy gap of a regioncontaining less nitrogen (the oxide semiconductor layer 303 c); thus,carriers easily flow through the region. Much nitrogen can be containedin a region of the oxide semiconductor layer, where carriers flow, andless nitrogen can be contained in other regions by forming the oxidesemiconductor layer 303 d having a high nitrogen concentration and theoxide semiconductor layer 303 c in such a manner that the kinds of gasesintroduced into the film formation chamber are switched and the filmformation is performed during heating.

At the formation of the oxide semiconductor layer 303 d having a highnitrogen concentration, the gas flow rate of nitrogen which isintroduced at the film formation or the like is controlled, whereby thecontrol of the degree of crystallinity, the crystal structure, and thevarious parameters involved in the crystal structure can be changed.

A crystal contained in the oxide semiconductor layer 303 d having a highnitrogen concentration has a hexagonal wurtzite crystal structure, and acrystal contained in the oxide semiconductor layer 303 c has a hexagonalnon-wurtzite crystal structure. Since both the wurtzite crystalstructure and the non-wurtzite crystal structure are hexagonal crystalstructures, a hexagonal lattice image can be seen from the c-axisdirection.

That is, a region 314 in the vicinity of the interface between the gateinsulating layer 302 a to which nitrogen is added and the oxidesemiconductor layer 303 has higher crystallinity than other regions.Further, the region 314 in the vicinity of the interface has higheruniformity than other regions. Furthermore, interface states due todangling bonds at the interface between the gate insulating layer 302and the oxide semiconductor layer 303 are reduced; therefore, afavorable condition of the interface can be realized.

Next, a conductive film is formed over the oxide semiconductor layer303. The conductive film can be formed by a sputtering method or avacuum evaporation method. As a material used for the conductive film, ametal material such as Al, Cu, Cr, Ta, Ti, Mo, and W, or an alloymaterial containing the metal material as a component is used. Further,a structure may be employed in which a layer of a high-melting pointmetal such as Cr, Ta, Ti, Mo, or W is stacked on a lower side and/or anupper side of a metal layer of Al, Cu, or the like.

Next, a source electrode layer 304 or a drain electrode layer 305 can beformed in such a manner that a mask having a desired shape is formedover the conductive film by a photolithography technique or an ink jetmethod, and the conductive film is selectively etched using the mask.Note that at the etching, part of the oxide semiconductor layer 303 maybe etched.

After the source electrode layer 304 or the drain electrode layer 305 isformed, heat treatment is performed under a reduced pressure. By theheat treatment under a reduced pressure, excessive hydrogen (includingwater and a hydroxyl group) contained in the oxide semiconductor layer303 can be removed. After the heat treatment under a reduced pressure isperformed, oxygen plasma treatment is performed. Oxygen is added to thevicinity of the surface of the exposed oxide semiconductor layer 303 byperforming the oxygen plasma treatment. The oxygen concentration of anoxide semiconductor layer 303 a to which oxygen is added is higher thanthe oxygen concentration of the oxide semiconductor layer 303 c to whichoxygen is not added.

As illustrated in FIG. 8C, after the oxygen plasma treatment isperformed, a protective insulating layer 306 which covers the sourceelectrode layer 304 or the drain electrode layer 305 and is in contactwith part of the oxide semiconductor layer 303 is formed by a sputteringmethod. The protective insulating layer 306 from which part of containedoxygen is released by heat treatment is easily formed by a sputteringmethod, which is preferable. When the protective insulating layer 306from which part of contained oxygen is released by heat treatment isformed by a sputtering method, the amount of oxygen contained in a filmformation gas is preferably large, and oxygen, a mixed gas of oxygen anda rare gas, or the like can be used. Typically, the oxygen concentrationof a film formation gas is preferably higher than or equal to 6% andlower than or equal to 100%.

As the protective insulating layer 306, a stacked-layer structureincluding one or more of silicon oxide, silicon oxynitride, siliconnitride, silicon nitride oxide, aluminum oxide, aluminum oxynitride,aluminum gallium oxide, and gallium oxide can be used.

Note that the thickness of the protective insulating layer 306 isgreater than or equal to 50 nm, preferably greater than or equal to 200nm and less than or equal to 500 nm. With the thick protectiveinsulating layer 306, the amount of oxygen released from the protectiveinsulating layer 306 can be increased, and thus the increase makes itpossible to reduce defects at the interface between the protectiveinsulating layer 306 and the oxide semiconductor layer 303.

After the protective insulating layer 306 is formed, if necessary, heattreatment may be performed under an atmosphere which hardly containshydrogen and moisture (a nitrogen atmosphere, an oxygen atmosphere, adry-air atmosphere (e.g., as for moisture, a dew point is lower than orequal to −40° C., preferably lower than or equal to −60° C.), or thelike) at a temperature higher than or equal to 150° C. and lower than orequal to 650° C., preferably a temperature higher than or equal to 200°C. and lower than or equal to 500° C.

Through the above process, a bottom-gate transistor 313 in which thecondition of the interface between the oxide semiconductor layer and theinsulating layer in contact with the oxide semiconductor layer isfavorable can be manufactured.

Next, the state of the nitrogen concentration of the oxide semiconductorlayer in the vicinity of the interface with the insulating layer incontact with the oxide semiconductor layer will be described in detailwith reference to the nitrogen concentration profile illustrated in FIG.9. FIG. 9 is a schematic diagram illustrating the nitrogen concentrationprofile in the film thickness direction of a cross section taken along adotted line in FIG. 8C. A first insulating layer and a second insulatinglayer in FIG. 9 correspond to the gate insulating layer and theprotective insulating layer in FIG. 8C, respectively.

As illustrated in FIG. 9, the value of the nitrogen concentration of theoxide semiconductor layer 303 c does not fluctuate much and ismaintained substantially constant.

The nitrogen concentration of the region 314 in the vicinity of theinterface is increased stepwise. Specifically, the nitrogenconcentration of the region 314 in the vicinity of the interface isincreased in two phases as closer to the first insulating layer. It isthe interface between the oxide semiconductor layer 303 c and the oxidesemiconductor layer 303 d having a high nitrogen concentration at whichthe nitrogen concentration is increased at first. The value of thenitrogen concentration which is once increased does not fluctuate muchand is maintained substantially constant. Further, the oxidesemiconductor layer 303 d having a high nitrogen concentration is incontact with the gate insulating layer 302 a to which nitrogen is added;therefore, regions in which the progress speed of crystallization differare formed in the oxide semiconductor layer 303 d having a high nitrogenconcentration. Thus, it is the interfaces between the regions in whichthe progress speed of crystallization differ in the oxide semiconductorlayer 303 d having a high nitrogen concentration at which the nitrogenconcentration is increased the second.

The progress speed of crystallization differs in the oxide semiconductorlayer 303 d having a high nitrogen concentration because the oxidesemiconductor layer 303 d having a high nitrogen concentration is not incontact with the gate insulating layer 302 b to which nitrogen is notadded but is in contact with the gate insulating layer 302 a to whichnitrogen is added. When the oxide semiconductor layer 303 d having ahigh nitrogen concentration is formed over and in contact with the gateinsulating layer 302 a to which nitrogen is added, there is a differencein the progress speed of crystallization in the oxide semiconductorlayer 303 d having a high nitrogen concentration.

As compared to the interface between the oxide semiconductor layer andthe first insulating layer in FIG. 3B, the interface between the oxidesemiconductor layer and the first insulating layer in FIG. 9 containsmuch nitrogen and thus crystallization progresses easily.

The nitrogen concentration has a peak at the interface between the oxidesemiconductor layer and the first insulating layer; therefore, thecrystallinity is the highest at the interface. In addition, the peakvalue of the nitrogen concentration is higher than the peak value of thenitrogen concentration in FIG. 3B. As a result, interface states due todangling bonds at the interface between the oxide semiconductor layerand the first insulating layer are reduced; therefore, a favorablecondition of the interface can be realized. Thus, as compared to thecase where the interface between the oxide semiconductor layer and thefirst insulating layer is in an amorphous state, the electriccharacteristics of the transistor can be prevented from being degraded.

Further, in the transistor 313 having the nitrogen concentration profileillustrated in FIG. 9, the nitrogen concentration of the region 314 inthe vicinity of the interface between the first insulating layer and theoxide semiconductor layer is high, and further the nitrogenconcentration at the interface between the first insulating layer andthe oxide semiconductor layer is much higher. Thus, the region 314 inthe vicinity of the interface has higher crystallinity than otherregions and is also c-axis aligned. Therefore, the transistor 313 withhigher electric characteristics (e.g., the field-effect mobility and thethreshold value) can be obtained. The region 314 in the vicinity of theinterface also has higher uniformity than other regions; therefore, thetransistor 313 with small variation in electric characteristics can beobtained.

Further, the transistor 313 is manufactured in which a channel region isincluded in a crystalline oxide semiconductor film having an a-b planewhere bonds for forming hexagonal lattices are formed, which issubstantially parallel to the plan surface of the substrate, and c-axeswhich are substantially perpendicular to the plan surface of thesubstrate, whereby the amount of fluctuation in the threshold voltage ofthe transistor 313 between before and after performance of abias-thermal stress (BT) test or light irradiation test on thetransistor 313 can be reduced. Accordingly, the transistor 313 withstable electric characteristics can be manufactured.

Embodiment 4

In this embodiment, an example of manufacturing a display device inwhich at least part of a driver circuit and a transistor to be disposedin a pixel portion are formed over the same substrate will be describedbelow.

The transistor to be disposed in the pixel portion is formed accordingto any one of Embodiments 1 to 3. Further, the transistor described inany of Embodiments 1 to 3 is an n-channel transistor, and thus part of adriver circuit that can be formed with n-channel transistors amongdriver circuits is formed over the same substrate as the transistor ofthe pixel portion.

FIG. 5A is an example of a block diagram of an active matrix displaydevice. Over a substrate 5300 in the display device, a pixel portion5301, a first scan line driver circuit 5302, a second scan line drivercircuit 5303, and a signal line driver circuit 5304 are provided. In thepixel portion 5301, a plurality of signal lines extended from the signalline driver circuit 5304 is arranged and a plurality of scan linesextended from the first scan line driver circuit 5302 and the secondscan line driver circuit 5303 is arranged. Note that pixels whichinclude display elements are provided in a matrix in respective regionswhere the scan lines and the signal lines intersect with each other.Further, the substrate 5300 in the display device is connected to atiming control circuit (also referred to as a controller or a controllerIC) through a connection portion such as a flexible printed circuit(FPC).

In FIG. 5A, the first scan line driver circuit 5302, the second scanline driver circuit 5303, and the signal line driver circuit 5304 areformed over the same substrate 5300 as the pixel portion 5301.Accordingly, the number of components such as a driver circuit which areprovided outside are reduced, so that reduction in cost can be achieved.Further, if the driver circuit is provided outside the substrate 5300,wirings would need to be extended and the number of connections ofwirings would be increased, but by providing the driver circuit over thesubstrate 5300, the number of connections of the wirings can be reduced.Accordingly, improvement in reliability and yield can be achieved.

FIG. 5B shows an example of a circuit configuration of the pixelportion. Here, a pixel configuration of a VA liquid crystal displaypanel is described.

In this pixel configuration, a plurality of pixel electrode layers isprovided in one pixel, and the pixel electrode layers are connected torespective transistors. The transistors are constructed so as to bedriven by different gate signals. In other words, signals applied toindividual pixel electrode layers in a multi-domain pixel are controlledindependently.

A gate wiring 602 of a transistor 628 and a gate wiring 603 of atransistor 629 are separated so that different gate signals can be giventhereto. In contrast, a source or drain electrode layer 616 functioningas a data line is shared by the transistor 628 and the transistor 629.As each of the transistor 628 and the transistor 629, any of thetransistors described in Embodiments 1 to 3 can be used as appropriate.

A first pixel electrode layer and a second pixel electrode layer havedifferent shapes and are separated by a slit. The second pixel electrodelayer is provided so as to surround the external side of the first pixelelectrode layer which is spread in a V shape. Timing of voltageapplication is made to vary between the first pixel electrode layer andthe second pixel electrode layer by the transistor 628 and thetransistor 629 in order to control alignment of the liquid crystal. Thetransistor 628 is connected to the gate wiring 602, and the transistor629 is connected to the gate wiring 603. When different gate signals aresupplied to the gate wiring 602 and the gate wiring 603, operationtimings of the transistor 628 and the transistor 629 can be varied.

Further, a storage capacitor is formed using a capacitor wiring 690, agate insulating layer as a dielectric, and a capacitor electrodeelectrically connected to the first pixel electrode layer or the secondpixel electrode layer.

The first pixel electrode layer, a liquid crystal layer, and a counterelectrode layer overlap each other to form a first liquid crystalelement 651. The second pixel electrode layer, a liquid crystal layer,and a counter electrode layer overlap each other to form a second liquidcrystal element 652. The pixel structure is a multi-domain structure inwhich the first liquid crystal element 651 and the second liquid crystalelement 652 are provided in one pixel.

Note that the pixel configuration is not limited to the structure shownin FIG. 5B. For example, a switch, a resistor, a capacitor, atransistor, a sensor, a logic circuit, or the like may be added to thepixel illustrated in FIG. 5B.

In this embodiment, an example of the VA liquid crystal display panel isshown; however, it is not particularly limited, and the presentinvention can be applied to various modes of liquid crystal displaydevices. For example, as a method for improving viewing anglecharacteristics, the present invention can be applied to a lateralelectric field method (also referred to as IPS) in which an electricfield in the horizontal direction to the main surface of the substrateis applied to the liquid crystal layer.

For example, it is preferable to use a liquid crystal phase exhibiting ablue phase for which an alignment film is not necessary for an IPSliquid crystal display panel. A blue phase is one of liquid crystalphases, which is generated just before a cholesteric phase changes intoan isotropic phase while temperature of cholesteric liquid crystal isincreased. Since the blue phase appears only in a narrow temperaturerange, a liquid crystal composition in which a chiral agent at 5 wt % ormore is mixed is used for the liquid crystal layer of the liquid crystalelement in order to widen the temperature range. The liquid crystalcomposition which includes a liquid crystal phase exhibiting a bluephase and a chiral agent has a short response time of shorter than orequal to 1 msec, and has optical isotropy, which makes the alignmentprocess unneeded and viewing angle dependence small.

Further, as a technique for improving moving image characteristics of aliquid crystal display device, there is another driving technique (e.g.,a field sequential method) in which, as a backlight, a surface lightsource including a plurality of LED (light-emitting diode) light sourcesor a plurality of EL light sources is used, and each light sourceincluded in the surface light source is independently driven so as toperform intermittent lighting in one frame period. As the surface lightsource, three or more kinds of LEDs may be used or an LED emitting whitelight may be used. In the case where three or more kinds of lightsources emitting different colors (e.g., light sources of red (R), green(G), and blue (B)) are used as the surface light source, color displaycan be performed without a color filter. Further, in the case where anLED emitting white light is used as the surface light source, colordisplay is performed with a color filter. Since a plurality of LEDs canbe controlled independently, the light emission timing of LEDs can besynchronized with the timing at which a liquid crystal layer isoptically modulated. Part of the LEDs can be turned off; therefore, anadvantageous effect of reducing power consumption can be obtainedparticularly in the case where an image having a large black part isdisplayed.

FIG. 5C shows another example of a circuit configuration of the pixelportion. Here, a pixel structure of a display panel using an organic ELelement is described.

In an organic EL element, by application of voltage to a light-emittingelement, electrons and holes are separately injected from a pair ofelectrodes into a layer containing a light-emitting organic compound,and current flows. The carriers (electrons and holes) are recombined,and thus, the light-emitting organic compound is excited. Thelight-emitting organic compound returns to a ground state from theexcited state, thereby emitting light. Owing to such a mechanism, thislight-emitting element is referred to as a current-excitationlight-emitting element.

FIG. 5C shows an example of a pixel configuration to which digital timegrayscale driving can be applied as an example of a semiconductordevice.

A structure and operation of a pixel to which digital time grayscaledriving can be applied are described. Here, one pixel includes twon-channel transistors each of which includes an oxide semiconductorlayer as a channel formation region.

A pixel 6400 includes a switching transistor 6401, a driver transistor6402, a light-emitting element 6404, and a capacitor 6403. A gateelectrode layer of the switching transistor 6401 is connected to a scanline 6406, a first electrode (one of a source electrode layer and adrain electrode layer) of the switching transistor 6401 is connected toa signal line 6405, and a second electrode (the other of the sourceelectrode layer and the drain electrode layer) of the switchingtransistor 6401 is connected to a gate electrode layer of the drivertransistor 6402. The gate electrode layer of the driver transistor 6402is connected to a power supply line 6407 through the capacitor 6403, afirst electrode of the driver transistor 6402 is connected to the powersupply line 6407, and a second electrode of the driver transistor 6402is connected to a first electrode (a pixel electrode) of thelight-emitting element 6404. A second electrode of the light-emittingelement 6404 corresponds to a common electrode 6408. The commonelectrode 6408 is electrically connected to a common potential lineprovided over the same substrate.

The second electrode (common electrode 6408) of the light-emittingelement 6404 is set to a low power supply potential. Note that the lowpower supply potential is a potential that is lower than a high powersupply potential with reference to the high power supply potential thatis set to the power supply line 6407. As the low power supply potential,GND, 0 V, or the like may be employed, for example. A potentialdifference between the high power supply potential and the low powersupply potential is applied to the light-emitting element 6404 andcurrent is supplied to the light-emitting element 6404, so that thelight-emitting element 6404 emits light. Here, in order to make thelight-emitting element 6404 emit light, each potential is set so thatthe potential difference between the high power supply potential and thelow power supply potential is higher than or equal to a forwardthreshold voltage of the light-emitting element 6404.

Note that gate capacitance of the driver transistor 6402 may be used asa substitute for the capacitor 6403, so that the capacitor 6403 can beomitted. The gate capacitance of the driver transistor 6402 may beformed between the channel formation region and the gate electrodelayer.

In the case of a voltage-input voltage driving method, a video signal isinput to the gate electrode layer of the driver transistor 6402 so thatthe driver transistor 6402 is in either of two states of beingsufficiently turned on and turned off. That is, the driver transistor6402 operates in a linear region. Thus, voltage higher than the voltageof the power supply line 6407 is applied to the gate electrode layer ofthe driver transistor 6402. Note that a voltage higher than or equal tothe sum of the voltage of the power supply line and Vth of the drivertransistor 6402 is applied to the signal line 6405.

In the case where analog grayscale driving is performed instead ofdigital time grayscale driving, the same pixel configuration as FIG. 5Ccan be employed by inputting signals in a different way.

In the case where analog grayscale driving is performed, a voltagegreater than or equal to the sum of the forward voltage of thelight-emitting element 6404 and Vth of the driving transistor 6402 isapplied to the gate electrode layer of the driver transistor 6402. Theforward voltage of the light-emitting element 6404 indicates a voltageat which a desired luminance is obtained, and includes at least forwardthreshold voltage. The video signal by which the driver transistor 6402operates in a saturation region is input, so that current can besupplied to the light-emitting element 6404. In order for the drivertransistor 6402 to operate in the saturation region, the potential ofthe power supply line 6407 is set higher than the gate potential of thedriver transistor 6402. When an analog video signal is used, it ispossible to feed current to the light-emitting element 6404 inaccordance with the video signal and perform analog grayscale driving.

Note that the pixel configuration is not limited to the structure shownin FIG. 5C. For example, a switch, a resistor, a capacitor, atransistor, a sensor, a logic circuit, or the like may be added to thepixel illustrated in FIG. 5C.

Next, the structure of a light-emitting element will be described withreference to FIGS. 6A to 6C. A cross-sectional structure of a pixel willbe described by taking an n-channel transistor for driving alight-emitting element as an example. A transistor 7001 for driving alight-emitting element, a transistor 7011 for driving a light-emittingelement, and a transistor 7021 for driving a light-emitting elementwhich are used for semiconductor devices illustrated in FIGS. 6A, 6B,and 6C, respectively, can be manufactured in a manner similar to that ofthe transistor described in any of Embodiments 1 to 3, and in each ofthese transistors, nitrogen is contained at a high concentration at aninterface between a gate insulating layer and an oxide semiconductorlayer.

At least one of the first electrode and the second electrode of thelight-emitting element is formed using a conductive film which transmitsvisible light, and light emission is extracted from the light-emittingelement. Examples of the structures of the light-emitting elementfocusing on directions from which light is extracted are a top emissionstructure in which light emission is extracted from the side of thesubstrate on which the light-emitting element is formed without passingthrough the substrate over which the light-emitting element and thetransistor are formed; a bottom emission structure in which lightemission is extracted from the side where the light-emitting element isnot formed through the substrate over which the light-emitting elementis formed; and a dual emission structure in which light emission isextracted to both the side of the substrate on which the light-emittingelement is formed and the other side of the substrate on which thelight-emitting element is not formed. The pixel configurationillustrated in FIG. 5C can be applied to a light-emitting element havingany of these emission structures.

A light-emitting element having a bottom emission structure is describedwith reference to FIG. 6A. The light-emitting element having a bottomemission structure emits light in a direction indicated by arrows inFIG. 6A.

In FIG. 6A, an example in which the transistor 211 described inEmbodiment 2 is used as the transistor 7011 for driving a light-emittingelement is shown; however, there is no particular limitation.

In FIG. 6A, over a first electrode 7017 having a light-transmittingproperty which is electrically connected to a source electrode or adrain electrode of the driver transistor 7011 for driving alight-emitting element, an EL layer 7014, a second electrode 7015, and alight shielding film 7016 are stacked in this order.

The first electrode 7017 is formed using a conductive film whichtransmits visible light. For the conductive film which transmits visiblelight, for example, indium oxide containing tungsten oxide, indium zincoxide containing tungsten oxide, indium oxide containing titanium oxide,indium tin oxide containing titanium oxide, indium tin oxide(hereinafter referred to as ITO), indium zinc oxide, and indium tinoxide to which silicon oxide is added can be given. Further, a metalthin film with a thickness enough to transmit light (preferably,approximately 5 nm to 30 nm) can also be used. For example, an aluminumfilm with a thickness of 20 nm can be stacked over another conductivefilm having a light-transmitting property.

As for the second electrode 7015, a material which efficiently reflectslight emitted from the EL layer 7014 is preferably used, in which casethe light extraction efficiency can be improved. Note that the secondelectrode 7015 may have a stacked-layer structure. For example, it isalso possible to stack a conductive film which transmits visible lighton the side which is in contact with the EL layer 7014 and stack thelight shielding film 7016 as film which shields light on the other sideof the conductive film which transmits visible light. As the lightshielding film, although a metal film or the like which efficientlyreflects light emitted from the EL layer is preferable, for example, aresin or the like to which a black pigment is added can also be used.

Note that one of the first electrode 7017 and the second electrode 7015functions as an anode, and the other functions as a cathode. It ispreferable to use a substance having a high work function for theelectrode which functions as an anode, and a substance having a low workfunction for the electrode which functions as a cathode.

As a material having a high work function, for example, ZrN, Ti, W, Ni,Pt, Cr, ITO, or IZO (registered trademark) can be used. As a materialhaving a low work function, for example, an alkali metal such as Li orCs, an alkaline earth metal such as Mg, Ca, or Sr, an alloy includingany of these (such as Mg:Ag or Al:Li), a rare earth metal such as Yb orEr, or the like can be used.

Note that when power consumption is compared, it is preferable that thefirst electrode 7017 serve as a cathode and the second electrode 7015serve as an anode because an increase in voltage of a driver circuitportion can be suppressed and power consumption can be reduced.

The EL layer 7014 includes at least the light-emitting layer and may beeither a single layer or a stack of plural layers. As the structure inwhich a plurality of layers is stacked, a structure in which an anode, ahole injection layer, a hole transport layer, a light-emitting layer, anelectron transport layer, an electron injection layer, and a cathode arestacked in this order can be given as an example. Note that unlike thelight-emitting layer, not all of these layers except the light-emittinglayer are necessarily provided in the EL layer 7014. Further, each ofthese layers may be provided in duplicate or more. Specifically, in theEL layer 7014, a plurality of light-emitting layers may be overlappedeach other or another hole injection layer may overlap the electroninjection layer. Furthermore, another component such as anelectron-relay layer may be added as appropriate as an intermediatelayer, in addition to the charge generation layer.

A light-emitting element 7012 is provided with a partition wall 7019 tocover edges of the first electrode 7017. As the partition wall 7019, aninorganic insulating film or an organic polysiloxane film can be appliedin addition to an organic resin film of polyimide, acrylic, polyamide,epoxy, or the like. It is particularly preferable that the partitionwall 7019 be formed using a photosensitive resin material so that a sidesurface of the partition wall 7019 be formed as a tilted surface with acontinuous curvature. In the case where a photosensitive resin materialis used for the partition wall 7019, a step of forming a resist mask canbe omitted. Further, the partition wall can be formed using an inorganicinsulating film. When the inorganic insulating film is used for thepartition wall, the amount of moisture contained in the partition wallcan be reduced.

Note that a color filter layer 7033 is provided between thelight-emitting element 7012 and a substrate 7010 (see FIG. 6A). Astructure for emitting white light is employed for the light-emittingelement 7012, whereby light emitted from the light-emitting element 7012passes through the color filter layer 7033 and then passes through asecond gate insulating layer 7031, a first gate insulating layer 7030,and the substrate 7010 so as to be emitted to the outside.

Plural kinds of the color filter layer 7033 may be formed. For example,a red color filter layer, a blue color filter layer, a green colorfilter layer, or the like can be provided in each pixel. Note that thecolor filter layer 7033 is formed by a droplet discharge method such asan ink jet method, a printing method, an etching method using aphotolithography technique, or the like.

The color filter layer 7033 is covered with an overcoat layer 7034 and aprotective insulating layer 7035 is further formed thereover. Note thatalthough the overcoat layer 7034 with a small thickness is illustratedin FIG. 6A, the overcoat layer 7034 is formed using a resin materialsuch as an acrylic resin and has a function of planarizing roughness dueto the color filter layer 7033.

A contact hole which is formed in the second gate insulating layer 7031,an insulating layer 7032, the color filter layer 7033, the overcoatlayer 7034, and the protective insulating layer 7035 and which reachesthe drain electrode layer is provided in a position overlapping thepartition wall 7019.

Next, a light-emitting element having a dual emission structure isdescribed with reference to FIG. 6B. The light-emitting element having adual emission structure emits light in a direction indicated by arrowsin FIG. 6B.

In FIG. 6B, an example in which the transistor 211 described inEmbodiment 2 is used as the transistor 7021 for driving a light-emittingelement is shown; however, there is no particular limitation.

In FIG. 6B, over a first electrode 7027 having a light-transmittingproperty which is electrically connected to a source electrode or adrain electrode of the driver transistor 7021 for driving alight-emitting element, an EL layer 7024 and a second electrode 7025 arestacked in this order.

The first electrode 7027 and the second electrode 7025 are each formedusing a conductive film which transmits visible light. The materialwhich can be used for the first electrode 7017 of FIG. 6A can be usedfor the conductive film which transmits visible light. Thus, thedescription of the first electrode 7017 is referred to for the details.

Note that one of the first electrode 7027 and the second electrode 7025functions as an anode, and the other functions as a cathode. It ispreferable to use a substance having a high work function for theelectrode which functions as an anode, and a substance having a low workfunction for the electrode which functions as a cathode.

The EL layer 7024 may be either a single layer or a stack of plurallayers. As for the EL layer 7024, the structure and material which canbe used in FIG. 6A for the EL layer 7014 can be used. Thus, thedescription of the EL layer 7014 is referred to for the details.

A light-emitting element 7022 is provided with a partition wall 7029 tocover edges of the first electrode 7027. As for the partition wall 7029,the structure and material which can be used in FIG. 6A for thepartition wall 7019 can be used. Thus, the description of the partitionwall 7019 is referred to for the details.

In addition, in the element structure illustrated in FIG. 6B, light isemitted from the light-emitting element 7022 to both the secondelectrode 7025 side and the first electrode 7027 side as indicated byarrows, and light emitted to the first electrode 7027 side passesthrough a second gate insulating layer 7041, an insulating layer 7042, afirst gate insulating layer 7040, and a substrate 7020 so as to beemitted to the outside.

In the structure of FIG. 6B, when full color display is performed, forexample, the light-emitting element 7022 is used as a greenlight-emitting element, one of adjacent light-emitting elements is usedas a red light-emitting element, and the other is used as a bluelight-emitting element. Alternatively, a light-emitting display devicecapable of full color display may be manufactured using four kinds oflight-emitting elements which include a white light-emitting element inaddition to three kinds of light-emitting elements.

Next, a light-emitting element having a top emission structure isdescribed with reference to FIG. 6C. The light-emitting element having atop emission structure emits light in a direction indicated by arrows inFIG. 6C.

In FIG. 6C, an example in which the transistor 211 described inEmbodiment 2 is used as the transistor 7001 for driving a light-emittingelement is shown; however, there is no particular limitation.

In FIG. 6C, over a first electrode 7003 which is electrically connectedto a source electrode or a drain electrode of the driver transistor 7001for driving a light-emitting element, an EL layer 7004 and a secondelectrode 7005 are stacked in this order.

As for the first electrode 7003, a material which efficiently reflectslight emitted from the EL layer 7004 is preferably used, in which casethe light extraction efficiency can be improved. Note that the firstelectrode 7003 may have a stacked-layer structure. For example, it isalso possible to stack a conductive film which transmits visible lighton the side which is in contact with the EL layer 7004 and stack a lightshielding film on the other side of the conductive film which transmitsvisible light. As the light shielding film, although a metal film or thelike which efficiently reflects light emitted from the EL layer ispreferable, for example, a resin or the like to which a black pigment isadded can also be used.

The second electrode 7005 is formed using a conductive film whichtransmits visible light. The material which can be used for the firstelectrode 7017 of FIG. 6A can be used for the conductive film whichtransmits visible light. Thus, the description of the first electrode7017 is referred to for the details.

Note that one of the first electrode 7003 and the second electrode 7005functions as an anode, and the other functions as a cathode. It ispreferable to use a substance having a high work function for theelectrode which functions as an anode, and a substance having a low workfunction for the electrode which functions as a cathode.

The EL layer 7004 may be either a single layer or a stack of plurallayers. As for the EL layer 7004, the structure and material which canbe used in FIG. 6A for the EL layer 7014 can be used. Thus, thedescription of the EL layer 7014 is referred to for the details.

A light-emitting element 7002 is provided with a partition wall 7009 tocover edges of the first electrode 7003. As for the partition wall 7009,the structure and material which can be used in FIG. 6A for thepartition wall 7019 can be used. Thus, the description of the partitionwall 7019 is referred to for the details.

In FIG. 6C, the source electrode or the drain electrode of thetransistor 7001 for driving a light-emitting element is electricallyconnected to the first electrode 7003 through a contact hole provided ina protective insulating layer 7052 and an insulating layer 7055. Aplanarizing insulating layer 7053 can be formed using a resin materialsuch as polyimide, acrylic, benzocyclobutene resin, polyamide, or epoxy.Other than such organic materials, it is also possible to use alow-dielectric constant material (a low-k material), a siloxane-basedresin, or the like. Note that the planarizing insulating layer 7053 maybe formed by stacking a plurality of insulating films formed using anyof these materials. There is no particular limitation on the formationmethod of the planarizing insulating layer 7053, and the followingmethod can be employed depending on the material: a sputtering method,an SOG method, spin coating, dip coating, spray coating, a dropletdischarging method (e.g., an ink jet method, screen printing, or offsetprinting), or the like.

In the structure of FIG. 6C, when full color display is performed, forexample, the light-emitting element 7002 is used as a greenlight-emitting element, one of adjacent light-emitting elements is usedas a red light-emitting element, and the other is used as a bluelight-emitting element. Alternatively, a light-emitting display devicecapable of full color display may be manufactured using four kinds oflight-emitting elements which include a white light-emitting element inaddition to three kinds of light-emitting elements.

In the structure of FIG. 6C, a light-emitting display device capable offull color display may be manufactured in such a manner that all of aplurality of light-emitting elements which is arranged is whitelight-emitting elements and a sealing substrate having a color filter orthe like is provided over the light-emitting element 7002. When amaterial which exhibits monochromatic light such as white light isformed and combined with color filters or color conversion layers,full-color display can be performed.

Needless to say, display of monochromatic light can also be performed.For example, a lighting device may be formed utilizing white lightemission; alternatively, an area-color light-emitting device usingmonochromatic light emission may be formed.

If necessary, an optical film such as a polarizing film including acircularly polarizing plate may be provided.

Note that the example is described in which a transistor which controlsthe driving of a light-emitting element (a transistor for driving alight-emitting element) is electrically connected to the light-emittingelement; however, a structure may be employed in which a transistor forcurrent control is connected between the transistor for driving alight-emitting element and the light-emitting element.

Note that the structure of the semiconductor device described in thisembodiment is not limited to those illustrated in FIGS. 6A to 6C and canbe modified in various ways based on the spirit of techniques of thepresent invention.

Embodiment 5

A semiconductor device disclosed in this specification can be applied toa variety of electronic devices (including game machines). Examples ofelectronic devices are a television set (also referred to as atelevision or a television receiver), a monitor of a computer or thelike, a camera such as a digital camera or a digital video camera, adigital photo frame, a mobile phone handset (also referred to as amobile phone or a mobile phone device), a portable game machine, aportable information terminal, an audio reproducing device, alarge-sized game machine such as a pachinko machine, and the like.Examples of electronic devices each including the display devicedescribed in any of the above embodiments will be described.

FIG. 7A illustrates a portable information terminal, which includes amain body 3001, a housing 3002, a display portion 3003 a, a displayportion 3003 b, and the like. The display portion 3003 b functions as apanel having a touch input function. By touching keyboard buttons 3004displayed on the display portion 3003 b, a screen can operate, and textcan be input. Needless to say, the display portion 3003 a may functionas a panel having a touch input function. A liquid crystal panel or anorganic light-emitting panel described in Embodiment 4 is manufacturedby using the transistor 111 described in Embodiment 1 as a switchingelement and applied to the display portion 3003 a or 3003 b, whereby aportable information terminal can be provided.

The portable information terminal illustrated in FIG. 7A has a functionof displaying various kinds of data (e.g., a still image, a movingimage, and a text image) on the display portion, a function ofdisplaying a calendar, a date, the time, or the like on the displayportion, a function of operating or editing the data displayed on thedisplay portion, a function of controlling processing by various kindsof software (programs), and the like. Furthermore, an externalconnection terminal (e.g., an earphone terminal or a USB terminal), arecording medium insertion portion, and the like may be provided on theback surface or the side surface of the housing.

The portable information terminal illustrated in FIG. 7A may transmitand receive data wirelessly. Through wireless communication, desiredbook data or the like can be purchased and downloaded from an electronicbook server.

Further, in the portable information terminal illustrated in FIG. 7A,one of the two display portions, the display portion 3003 a and thedisplay portion 3003 b, can be detached, the case of which isillustrated in FIG. 7B. When the display portion 3003 a also functionsas a panel having a touch input function, further reduction in weightcan be achieved when the portable information terminal illustrated inFIG. 7A is carried. Accordingly, the housing 3002 can be held by onehand and can operate by the other hand, which is convenient.

Furthermore, when the housing 3002 illustrated in FIG. 7B functions asan antenna, a microphone, or a wireless communication device, thehousing 3002 may be used as a mobile phone handset.

FIG. 7C shows an example of a mobile phone handset. A mobile phonehandset 5005 illustrated in FIG. 7C is provided with a display portion5001 incorporated in a housing, a display panel 5003 attached to a hinge5002, operation buttons 5004, a speaker, a microphone, and the like.

In the mobile phone handset 5005 illustrated in FIG. 7C, the displaypanel 5003 is slid to overlap the display portion 5001, and the displaypanel 5003 also functions as a cover having a light-transmittingproperty. The display panel 5003 is a display panel including thelight-emitting element having a dual emission structure illustrated inFIG. 6B in Embodiment 4, in which light emission is extracted throughthe surface opposite to the substrate side and the surface on thesubstrate side.

Since the light-emitting element having a dual emission structure isused for the display panel 5003, display can be performed also with thedisplay portion 5001 overlapped; therefore, both the display portion5001 and the display panel 5003 can perform display and the users canview both the displays. The display panel 5003 has a light-transmittingproperty and the view beyond the display panel can be seen. For example,when a map is displayed on the display portion 5001 and the locationpoints of users are displayed using the display panel 5003, the presentlocation can be recognized easily.

Further, in the case where the mobile phone handset 5005 is providedwith an image sensor to be used as a television telephone, it ispossible to make conversation with plural persons while their faces aredisplayed; therefore, a television conference or the like can beperformed. For example, when the face of a single person or the faces ofplural persons are displayed on the display panel 5003 and further theface of another person is displayed on the display portion 5001, userscan make conversation while viewing the faces of two or more persons.

When a touch input button 5006 displayed on the display panel 5003 istouched with a finger or the like, data can be inputted into the mobilephone handset 5005. Further, the user can make a call or compose ane-mail by sliding the display panel 5003 and touching the operationbuttons 5004 with a finger or the like.

FIG. 7D shows an example of a television set. In a television set 9600,a display portion 9603 is incorporated in a housing 9601. The displayportion 9603 can display images. Here, the housing 9601 is supported ona stand 9605 provided with a CPU. When the transistor 211 described inEmbodiment 2 is applied to the display portion 9603, the television set9600 can be obtained.

The television set 9600 can operate by an operation switch of thehousing 9601 or a separate remote controller. Further, the remotecontroller may be provided with a display portion for displaying dataoutput from the remote controller.

Note that the television set 9600 is provided with a receiver, a modem,and the like. With the use of the receiver, general televisionbroadcasting can be received. Further, when the television device isconnected to a communication network with or without wires via themodem, one-way (from a sender to a receiver) or two-way (between asender and a receiver or between receivers) information communicationcan be performed.

Further, the television set 9600 is provided with an external connectionterminal 9604, a storage medium recording and reproducing portion 9602,and an external memory slot. The external connection terminal 9604 canbe connected to various types of cables such as a USB cable, and datacommunication with a personal computer or the like is possible. A diskstorage medium is inserted into the storage medium recording andreproducing portion 9602 and it is possible to read data stored in thestorage medium and write data to the storage medium. In addition, apicture, a video, or the like stored as data in an external memory 9606inserted to the external memory slot can be displayed on the displayportion 9603.

The structures, methods, and the like described in this embodiment canbe combined as appropriate with any of the structures, methods, and thelike described in the other embodiments.

Embodiment 6

In this embodiment, a semiconductor film will be described withreference to FIG. 10, FIGS. 11A and 11B, FIGS. 12A to 12C, FIGS. 13A and13B, and FIGS. 14A and 14B which includes a stack of an oxidesemiconductor film having a high nitrogen concentration, whose onesurface is in contact with an insulating surface, and an oxidesemiconductor film in contact with the other surface of the oxidesemiconductor film having a high nitrogen concentration; and the oxidesemiconductor film having a high nitrogen concentration includes ac-axis aligned wurtzite crystal (a first crystal type) and the oxidesemiconductor film includes a c-axis aligned hexagonal anisotropiccrystal (a second crystal type) different from the first crystal type.

A schematic cross-sectional view of a semiconductor film including astack of different crystal structures, an example of which is shown inthis embodiment, is illustrated in FIG. 10.

A semiconductor film 430 includes an oxide semiconductor film 431 havinga high nitrogen concentration and an oxide semiconductor film 432. Amode of the semiconductor film 430 in which the oxide semiconductor film431 having a high nitrogen concentration is provided in contact with aninsulating surface 402 over a substrate 400 is illustrated in FIG. 10.Note that the insulating surface 402 includes an insulating surface 402a to which nitrogen is added and an insulating surface 402 b to whichnitrogen is not added. The oxide semiconductor film 431 having a highnitrogen concentration includes a c-axis aligned wurtzite crystal as thefirst crystal, and the oxide semiconductor film 432 includes a c-axisaligned hexagonal anisotropic crystal as the second crystal.Accordingly, the oxide semiconductor film 431 having a high nitrogenconcentration has higher crystallinity than the oxide semiconductor film432.

<Hexagonal Crystal Structure>

Hexagonal crystal structures as the first crystal type and the secondcrystal type will be described first.

First, the wurtzite crystal structure as the first crystal type will bedescribed with reference to FIGS. 11A and 11B. As for the wurtzitecrystal structure, FIG. 11A illustrates distribution of atoms in the a-bplane, and FIG. 11B illustrates a structure where the c-axis directionis the vertical direction.

Indium nitride and gallium nitride can be given as examples of amaterial whose crystal has the wurtzite crystal structure. Further, anoxide semiconductor containing nitrogen can be a film including thec-axis aligned wurtzite crystal in some cases.

Specifically, an In—Ga—Zn—O film containing nitrogen at a concentrationhigher than or equal to 5×10¹⁹/cm³, preferably higher than or equal to1×10²⁰/cm³ and lower than 7 atomic %, becomes a film including thec-axis aligned wurtzite crystal; and In, Ga, and Zn are included atrandom in a metal site.

Next, the hexagonal crystal structure as the second crystal type will bedescribed.

For example, an In—Ga—Zn—O film containing nitrogen at a concentrationhigher than or equal to 1×10¹⁷/cm³ and lower than 5×10¹⁹/cm³ becomes afilm including the c-axis aligned hexagonal crystal as the secondcrystal type. The In—Ga—Zn—O film including the c-axis aligned hexagonalcrystal as the second crystal type has an In—O crystal plane (a crystalplane containing indium and oxygen) in the a-b plane and two layerscontaining Ga and Zn between In—O crystal planes. Note that as for thetwo layers containing Ga and Zn, there is no limitation on the positionof Ga and Zn as long as at least one of Ga and Zn is contained in eachof the layers.

The wurtzite crystal structure as the first crystal type and thehexagonal crystal structure as the second crystal type are bothhexagonal crystal structures, in which atoms are arranged in a hexagonalshape in the a-b plane. Further, the hexagonal crystal as the secondcrystal type is in contact with the wurtzite crystal, and the hexagonalcrystal as the second crystal type is matched with the wurtzite crystal.

FIGS. 12A to 12C illustrate a manner in which the hexagonal secondcrystal having the same lattice constant is aligned on the wurtzitecrystal. FIG. 12A illustrates the hexagonal second crystal structure,and FIG. 12B illustrates the wurtzite crystal structure. In addition,FIG. 12C is a schematic view illustrating a manner in which thehexagonal second crystal is in contact with the wurtzite crystal and thehexagonal second crystal is matched with the wurtzite crystal.

Thus, the oxide semiconductor film having a high nitrogen concentrationincluding the wurtzite crystal which has high crystallinity and iseasily crystallized is formed, and the oxide semiconductor film isformed in contact with the oxide semiconductor film having a highnitrogen concentration, whereby an advantageous effect that the wurtzitecrystal included in the oxide semiconductor film having a high nitrogenconcentration facilitates crystallization of the oxide semiconductorfilm is obtained.

<Oxide Semiconductor Film Having High Nitrogen Concentration>

Next, the oxide semiconductor film having a high nitrogen concentrationwill be described. The oxide semiconductor film having a high nitrogenconcentration includes the c-axis aligned wurtzite crystal. Inparticular, the oxide semiconductor film having a high nitrogenconcentration is formed using a material that has high crystallinity andis easily crystallized as compared to the oxide semiconductor film.

The wurtzite crystal as the first crystal type which can be applied tothe oxide semiconductor film having a high nitrogen concentration willbe described below.

The nitrogen concentration of the oxide semiconductor film having a highnitrogen concentration, which has a wurtzite crystal structure, ishigher than or equal to 5×10¹⁹/cm³, preferably higher than or equal to1×10²⁰/cm³ and lower than 7 atomic %. In an oxide semiconductor filmhaving a high nitrogen concentration in which nitrogen is intentionallycontained so that a nitrogen concentration higher than or equal to5×10¹⁹/cm³, preferably higher than or equal to 1×10²⁰/cm³ and lower than7 atomic %, can be obtained, the energy gap is smaller than the energygap of an oxide semiconductor film in which nitrogen is not containedintentionally; thus, carriers easily flow therethrough.

Note that a diffraction image where bright points appear alternately maybe observed in an observation image of the wurtzite crystal structure,which is obtained using a high-angle annular dark field (HAADF)-STEM.

FIG. 13A shows a HAADF-STEM observation image obtained by calculationbased on the wurtzite crystal structure.

FIG. 13B shows a HAADF-STEM observation image of an In—Ga—Zn—O filmformed using a deposition gas containing only nitrogen.

From each of the HAADF-STEM observation images in FIGS. 13A and 13B, itcan be confirmed that the wurtzite crystal structure has a two-cyclelayer structure.

Note that the In—Ga—Zn—O film containing nitrogen was formed by asputtering method over a quartz glass substrate to a thickness of 300nm. Deposition was performed under conditions where a target containingIn, Ga, and Zn at 1:1:1 [atomic ratio] was used, the distance betweenthe substrate and the target was 60 mm, a DC power source was used, thepower was 0.5 kw, and the pressure was 0.4 Pa. In addition, thesubstrate temperature during deposition was 400° C., and only nitrogenwas introduced as a sputtering gas into a deposition chamber at a flowrate of 40 sccm.

Next, the hexagonal crystal as the second crystal type will bedescribed.

As examples of the hexagonal (non-wurtzite) crystal structure as thesecond crystal type, a YbFe₂O₄-type structure, a Yb₂Fe₃O₇-typestructure, and deformed structures of the foregoing structures can begiven. For example, In—Ga—Zn—O that is a three-component metal oxide hasthe hexagonal crystal structure as the second crystal type and can beused for the oxide semiconductor film. Note that the In—Ga—Zn—O filmwhich can be used as the oxide semiconductor film may contain nitrogenat a concentration higher than or equal to 1×10¹⁷/cm³ and lower than orequal to 5×10¹⁹/cm³.

Examples of In—Ga—Zn—O that is a three-component metal oxide includeInGaZnO₄ having a YbFe₂O₄-type structure and In₂Ga₂ZnO₇ having aYb₂Fe₃O₇-type structure, and the In—Ga—Zn—O can have any of deformedstructures of the foregoing structures, which is disclosed in thefollowing document: M. Nakamura, N. Kimizuka, and T. Mohri, “The PhaseRelations in the In₂O₃—Ga₂ZnO₄—ZnO System at 1350° C.”, J. Solid StateChem., 1991, Vol. 93, pp. 298-315.

Further, as the oxide semiconductor film, a four-component metal oxidesuch as an In—Sn—Ga—Zn—O film; a three-component metal oxide such as anIn—Ga—Zn—O film, an In—Sn—Zn—O film, an In—Al—Zn—O film, a Sn—Ga—Zn—Ofilm, an Al—Ga—Zn—O film, or a Sn—Al—Zn—O film; a two-component metaloxide such as an In—Zn—O film, a Sn—Zn—O film, an Al—Zn—O film, or anIn—Ga—O film; or the like can be used. Further, silicon may be containedin the above oxide semiconductor film. In this specification, forexample, an In—Ga—Zn—O film means an oxide film containing indium (In),gallium (Ga), and zinc (Zn).

In addition, it is known that examples of the In—Ga—Zn—O that is athree-component metal oxide include InGaZnO₄ having a YbFe₂O₄-typestructure and In₂Ga₂ZnO₇ having a Yb₂Fe₃O₇-type structure, and theIn—Ga—Zn—O can have any of deformed structures of the foregoingstructures (M. Nakamura, N. Kimizuka, and T. Mohri, “The Phase Relationsin the In₂O₃—Ga₂ZnO₄—ZnO System at 1350° C.”, J. Solid State Chem.,1991, Vol. 93, pp. 298-315). Note that a layer containing Yb is denotedby an A layer and a layer containing Fe is denoted by a B layer, below.The YbFe₂O₄-type structure has a repeated structure of ABB|ABB|ABB. Asan example of a deformed structure of the YbFe₂O₄-type structure, arepeated structure of ABBB|ABBB can be given. Further, the Yb₂Fe₃O₇-typestructure has a repeated structure of ABB|AB|ABB|AB. As an example of adeformed structure of the Yb₂Fe₃O₇-type structure, a repeated structureof ABBB|ABB|ABBB|ABB|ABBB|ABB| can be given.

The oxide semiconductor film is formed over and in contact with theoxide semiconductor film having a high nitrogen concentration.Accordingly, a highly crystalline region of the semiconductor filmhaving a stacked-layer structure of different crystal structures is incontact with an insulating surface, whereby interface states due todangling bonds can be reduced, so that a semiconductor film which has astacked-layer structure of different crystal structures and a favorableinterface condition can be provided.

Note that a diffraction image where one layer with bright patternsappears in every third layer may be observed in an observation image ofthe hexagonal crystal structure as the second crystal type, which isobtained using a high-angle annular dark field (HAADF)-STEM.

FIG. 14A shows a HAADF-STEM observation image obtained by calculationbased on the hexagonal crystal structure as the second crystal type.

FIG. 14B shows a HAADF-STEM observation image of an In—Ga—Zn—O film.

From each of the HAADF-STEM observation images in FIGS. 14A and 14B, itcan be confirmed that one layer with bright patterns appears in everythird layer and that the hexagonal crystal structure as the secondcrystal type has a nine-cycle layer structure.

Note that the In—Ga—Zn—O film was formed by a sputtering method over aquartz glass substrate to a thickness of 300 nm. Deposition wasperformed under conditions where a target containing In, Ga, and Zn at1:1:1 [atomic ratio] was used, the distance between the substrate andthe target was 60 mm, a DC power source was used, the power was 0.5 kw,and the pressure was 0.4 Pa. In addition, the substrate temperatureduring deposition was 400° C., and only oxygen was introduced as asputtering gas into a deposition chamber at a flow rate of 40 sccm.

The structures, methods, and the like described in this embodiment canbe combined as appropriate with any of the structures, methods, and thelike described in the other embodiments.

This application is based on Japanese Patent Application Serial No.2010-267919 filed with Japan Patent Office on Nov. 30, 2010, the entirecontents of which are hereby incorporated by reference.

1. (canceled)
 2. A semiconductor device comprising: a gate electrodelayer; a gate insulating layer overlapping with the gate electrodelayer; a source electrode layer and a drain electrode layer in contactwith the gate insulating layer; an oxide semiconductor layer in contactwith the gate insulating layer, the source electrode layer, and thedrain electrode layer; and an insulating layer in contact with the oxidesemiconductor layer, wherein the oxide semiconductor layer has a peak ofa nitrogen concentration at an interface with the gate insulating layer,and wherein the gate insulating layer contains nitrogen and has a peakof a nitrogen concentration at an interface with the oxide semiconductorlayer.
 3. The semiconductor device according to claim 2, wherein thegate insulating layer is over the gate electrode layer, and wherein eachof the source electrode layer and the drain electrode layer is over theoxide semiconductor layer.
 4. The semiconductor device according toclaim 2, wherein the oxide semiconductor layer has a concentrationgradient of nitrogen, which becomes higher the closer to the gateinsulating layer, and has a concentration gradient of oxygen, whichbecomes higher the closer to the insulating layer.
 5. The semiconductordevice according to claim 2, wherein a region of the oxide semiconductorlayer, which is in the vicinity of the interface with the gateinsulating layer, has higher crystallinity than other regions.
 6. Thesemiconductor device according to claim 2, wherein a nitrogenconcentration of the oxide semiconductor layer in the vicinity of theinterface with the gate insulating layer is higher than or equal to5×10¹⁹/cm³.
 7. The semiconductor device according to claim 2, wherein anitrogen concentration of the oxide semiconductor layer in the vicinityof the interface with the insulating layer is lower than or equal to5×10¹⁹/cm³.
 8. The semiconductor device according to claim 2, whereinthe oxide semiconductor layer has a peak of an oxygen concentration atan interface with the insulating layer, and wherein the insulating layercontains oxygen and has a peak of an oxygen concentration at aninterface with the oxide semiconductor layer.
 9. The semiconductordevice according to claim 2, wherein at least part of the oxidesemiconductor layer has crystallinity and is c-axis aligned.
 10. Thesemiconductor device according to claim 2, wherein the oxidesemiconductor layer comprises indium and zinc.
 11. The semiconductordevice according to claim 2, wherein the oxide semiconductor layercomprises at least a hexagonal wurtzite crystal structure.
 12. Thesemiconductor device according to claim 2, wherein the oxidesemiconductor layer comprises indium, gallium and zinc.
 13. Thesemiconductor device according to claim 2, wherein the oxidesemiconductor layer includes a crystalline region the crystalline regionhaving a c-axis being substantially perpendicular to a surface of theoxide semiconductor layer.
 14. A semiconductor device comprising: anoxide semiconductor layer over an insulating layer; a source electrodelayer and a drain electrode layer over and in contact with the oxidesemiconductor layer; a gate insulating layer over and in contact withthe oxide semiconductor layer and over the source electrode layer andthe drain electrode layer; a gate electrode layer over the gateinsulating layer; wherein the oxide semiconductor layer has a peak of anitrogen concentration at an interface with the gate insulating layer,and wherein the gate insulating layer contains nitrogen and has a peakof a nitrogen concentration at an interface with the oxide semiconductorlayer.
 15. The semiconductor device according to claim 14, wherein theoxide semiconductor layer has a concentration gradient of nitrogen,which becomes higher the closer to the gate insulating layer, and has aconcentration gradient of oxygen, which becomes higher the closer to theinsulating layer.
 16. The semiconductor device according to claim 14,wherein a region of the oxide semiconductor layer, which is in thevicinity of the interface with the gate insulating layer, has highercrystallinity than other regions.
 17. The semiconductor device accordingto claim 14, wherein a nitrogen concentration of the oxide semiconductorlayer in the vicinity of the interface with the gate insulating layer ishigher than or equal to 5×10¹⁹/cm³.
 18. The semiconductor deviceaccording to claim 14, wherein a nitrogen concentration of the oxidesemiconductor layer in the vicinity of the interface with the insulatinglayer is lower than or equal to 5×10¹⁹/cm³.
 19. The semiconductor deviceaccording to claim 14, wherein the oxide semiconductor layer has a peakof an oxygen concentration at an interface with the insulating layer,and wherein the insulating layer contains oxygen and has a peak of anoxygen concentration at an interface with the oxide semiconductor layer.20. The semiconductor device according to claim 14, wherein at leastpart of the oxide semiconductor layer has crystallinity and is c-axisaligned.
 21. The semiconductor device according to claim 14, wherein theoxide semiconductor layer comprises indium and zinc.
 22. Thesemiconductor device according to claim 14, wherein the oxidesemiconductor layer comprises at least a hexagonal wurtzite crystalstructure.
 23. The semiconductor device according to claim 14, whereinthe oxide semiconductor layer comprises indium, gallium and zinc. 24.The semiconductor device according to claim 14, wherein the oxidesemiconductor layer includes a crystalline region, the crystallineregion having a c-axis being substantially perpendicular to a surface ofthe oxide semiconductor layer.